diff --git a/.editorconfig b/.editorconfig
index 89e6fb3..794098b 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -13,3 +13,6 @@ trim_trailing_whitespace = true
 
 [*.rb]
 indent_size = 2
+
+[*.ml]
+indent_size = 2
diff --git a/2021/README.md b/2021/README.md
index 2c0500c..5df7fb0 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -13,7 +13,7 @@
 | 07  |   ✓    |   ✓    | [Swift]  |
 | 08  |   ✓    |        | [C#]     |
 | 09  |   ✓    |   ✓    | [C]      |
-| 10  |        |        |          |
+| 10  |   ✓    |   ✓    | [OCaml]  |
 | 11  |        |        |          |
 | 12  |        |        |          |
 | 13  |        |        |          |
@@ -45,3 +45,4 @@
 [swift]: https://www.swift.org
 [c#]: https://docs.microsoft.com/en-us/dotnet/csharp/
 [c]: http://www.open-std.org/jtc1/sc22/wg14
+[ocaml]: https://ocaml.org
diff --git a/2021/day-10/.gitignore b/2021/day-10/.gitignore
new file mode 100644
index 0000000..156d937
--- /dev/null
+++ b/2021/day-10/.gitignore
@@ -0,0 +1,3 @@
+*.cmi
+*.cmx
+*.o
diff --git a/2021/day-10/Justfile b/2021/day-10/Justfile
new file mode 100644
index 0000000..9d6077d
--- /dev/null
+++ b/2021/day-10/Justfile
@@ -0,0 +1,6 @@
+@part PART INPUT_FILE="inputs/puzzle.txt":
+    ocamlopt -o part_{{PART}} common.ml part_{{PART}}.ml
+    ./part_{{PART}} {{INPUT_FILE}}
+
+clean:
+    rm -f *.cmx *.cmi *.o part_one part_two
diff --git a/2021/day-10/common.ml b/2021/day-10/common.ml
new file mode 100644
index 0000000..da4b090
--- /dev/null
+++ b/2021/day-10/common.ml
@@ -0,0 +1,21 @@
+let rec read_line in_chan =
+  try
+    let line = input_line in_chan in
+    line :: read_line in_chan
+  with End_of_file ->
+    close_in in_chan;
+    []
+
+let char_list str = List.init (String.length str) (String.get str)
+
+let read_file path = open_in path |> read_line |> List.map char_list
+
+let is_closing ch = List.mem ch [')'; ']'; '}'; '>']
+
+let is_pair o c =
+  match (o, c) with
+  | ('(', ')') -> true
+  | ('[', ']') -> true
+  | ('{', '}') -> true
+  | ('<', '>') -> true
+  | (_, _)     -> false
diff --git a/2021/day-10/inputs/puzzle.txt b/2021/day-10/inputs/puzzle.txt
new file mode 100644
index 0000000..97fdc78
--- /dev/null
+++ b/2021/day-10/inputs/puzzle.txt
@@ -0,0 +1,94 @@
+[{<<<{{[(<{{{{[]()}{[]()}}<[[]<>]<[]()>>}([[[][]]]<[{}<>]((){})>)}>)[<([<<(){}>{(){}}>[[<>{}](<>[
+<<{(<({[<{{<[<()()><{}{}>][[()[]][(){}]]>((<[]<>>(<>{}))((<>{}){<>()}))}<{({{}}[<>{}])({{}{}}{{}
+(<[(<<[((({<[{()[]}]>}{[[[{}[]]{{}{}}][{{}()}<<>()>]]})))<[([(<[{}{}]>{<<>{}>})<((<>()){[][]})<{<
+{({<[[[(<{{<<[[]()]>]}{<({<>{}}([]<>))[({}{}){()()}]>((<[][]>{[]()})[(()<>)[{}{}]])}}>){[<[{[{[]}{<><>
+(<{([(<[{[{<[<()<>>({}[])]>}][<{({<>}(<><>))}({[<>{}](<>())}{[{}()]<()<>>})><[<<{}<>><()[])>[<{}[]
+{<{[({<<<[[<(({}))>(([<><>][<><>])<<()()][{}{}]>)]][<[{<{}{}>(<>[])}](({{}[]}(()()))<<[]()>
+(<{[[(({(<([([[]{}][()[]]){{()[]}[<><>]}](<(<><>){[][]}>({{}{}}{{}()})))>){((([{()}[{}[]]]))[<[
+{<[(<([<{(<[[({}{})({}())]]>[(<<{}<>>([]())><({}[]}{()[]}>)([<(){}>][[{}<>][<><>]])])([{{<()()>[()[]]}[[<>](
+{(({([([<((<({<>{}}<[]>)(<[]{}>)>))({{{[<><>]<<><>>}[(<>{})[{}()]]}{[<[]{}>([]())]{({}{})}}}<({([]())}({<>()
+<<{<[<(([<[{[{[]{}}([][])]{{<><>}{<><>}}}{({<>[]})({<>[]})}]}<<[[[[]()]{[]()}]<{{}{}}[{}{}]>](<(()())[[][]]>[
+[{[<[{(([<<{{(()[])(<>[])}}{(<<>{}>([]<>))}]>({{((<><>)[[]{}])((()<>))}[[({}[])({}{})]([{}()]([]())
+{[{[{[{({([<{[<>()][(){}]}<[[]()]{(){}}>>]{{(([][])([]{}))}})}(<{{<({}())(<>[]>><[()()]>}{[((){}){<>{}}]
+[{{[<<[[(<({<{()()}(()[])>(<{}[]>)}[(<<>{}><[][]>)[({}<>]([]{})]])[<[<<>()><[]<>>]{<[][]>[<>]}>{[{{}()
+{[(([<({[<(({([][])<{}()>}{<{}[]><[]<>>))(<{[]<>}[()()]>))({{({}{})}<<<>[]>([][])>}[[{()[]}<<>{}>]{(<>[])
+{[<[(({{{{[((([]())[{}()])[[{}[])<{}<>>])((<()[]>{{}[]})(<<><>>(<>{})))]}}}[([[({[{}[]]<(){}>})(
+{[{((<[<{{([{([]())(<>{})}[<{}[]><[][]>]])<((<[]{}>{()[]})[(()())(<>())])(({{}()}([]())))>}}>[{[<<<((
+[{[[{{{<{<<({<{}{}>([][])}{([]{})[(){}]})<{(<>[]){()[]}}{[(){}][[]<>]}>>>}[[([([<>{}][[]{}])[
+[((<({<({([<(<<>()>)[{()<>}{[]<>}]>[[{{}[]}<{}()}]{{()[]}}]]{[<<[][]><<><>>><(<>)>]<{{()<>}[[]<>]}{(
+[<{[[[(<(({(<({}<>){<>[]}>){(<()<>>{()()})[{[]()}([]())]}}{[[<()[]){<>{}}]<<<>{}>[(){}]>]<[[()<>]<()[
+<{<<[[[[{[<<{({}<>){()[]}}([<>()]{<>})><(<<><>)<()<>>)[<[]()>[<>{}]]>>(<<({}{})[[][]]>([<>{}]({}<>))>{<<<
+{[[{<({<{(<[(([]{}){[]{}})]<[[[][]][{}{}]]{[{}<>}{{}<>}}>><({[()[]]<{}{}>}{({})})<<<()[]>[{}()]
+<[[{{([({[{(<{()()}>{(()[])(<>[])}){<[<>[]]{[]()}>[<[]<>>[<>()]]}}]})]<{{(([<<{}()>>(([]()){{}()})]<[(<>[]
+[(<<<({{<([{(((){})[{}[]])[{[]{}}(()<>)]}]{({{<>{}}[<><>]}{(()<>)})[[([]{})<()[]>]{(<>[]]<{}{}>}]})[{[[(()
+[<[{{[({{{({(<[][]>{[]})[{{}()}<[]{}>]}<[<<>()>({}{})]<{[][]}<[]{}>>>)}<[[{<<>()>(<>{})}<<()<>>[<>{}]>]]<<<
+<<<(<<<(<[{<(<(){}>((){}))({()<>}{<><>})>}([<({}())[<>()]><{{}()}([])>][{(())}(([])<<>{}>)})]>)>[[{((
+{[[(((<[{[{[<({}())((){})>[{<>{}}<<>{}>]]{{<<>()>{[]()}}<[()]({}())>}}(([{<><>}][<<>[]><[][]>])<[({}[]]]({
+{(<(({<({<[([<[][]><()()>]({{}{}}[{}<>]))<<(()[])[<>[]]>{{[]{}}(<>[])}>]((<[{}[]]>{<()()>[<>{
+<([{[([(<[([{<[]{}>[<>()]}[{<><>}]])[([([]<>){()[]}])[({{}[]})<[<>[]]<[]<>>>]]]>)])[(({<<{{[<>{}]{<>()}}}
+(<({{[<<{<{<{{()}{[]<>}]{{(){}}[[]()]}>{{{[][]}[()]}[(()<>)(<>[])]}}{{(<[]><(){}>){(<>())([])}}{[(<>)({}<>)
+{<[({{<<{([([((){})](<{}[]><<>{}>))([<<>[]><()()>])][<{{[]<>}(()<>)}<<()[]>{(){}}>>{(<()[]>[{}])
+[<[[{[{{[{(<[<()<>>]<[(){}][{}]>>)}([{{{(){}}[<>{}]}((()<>)<[]<>))}][({<[]{}>{[][]}}({{}{}
+{[[([<((<(([(<<><>>{[]()})]<{[()<>]{{}{}}}{[{}[]]<[]{}>}>)((<<[]<>>>({<>()}))(<[[]{}]>{{[]{}}[(
+{({[([{(<{{(<[<>[]]<[]<>>>(<{}[]>[<>[]]))<([{}<>]{{}()})<[()()]([]{})>>}(<{<<>{}>([][])}><(({}[])
+{<<{[([{({[[<<<>[]>[[]{}]>]({{<>[]}(()<>)}{[{}<>]<()[]>})]<({[()()][{}[]]}<<()()><(){}>>)>}[([
+<{{([<{{{{([(([]<>){<>{}})][<<<><>>[(){}]>])}{<([({}<>)<<>[]>])({{[]()}(()())){{<>[]}({}{})})>(<({<><>})><
+<({<{[<<({(<{[{}[]]{{}{}}}{[(){}][[]<>)}>[(<(){}><{}{}>){(<>)[[]{}]}])}(<[<(()())><<<>()>{()()}>][
+{[[<<{<({<[(([{}]<<>[]>))[<{<><>}({}{})>]]{([{()<>}{[]()}][{()()}<(){}>])(<({}{})<[]{}>>[[<>{}]
+{<[({{{[(<(({{()<>}(()[])}[<<>{}>{{}<>}])<{({}[])<()()>}{(()())<{}()>}>}([<(()())([][])>[([]){()<>}]]
+[{<[{{(([{{<({<>[]}(<>()))<[[][]]>>(([{}]({}())){[()[]]((){})})}{((<<>[]>[[]<>])<[<>[]}([]())>)
+[<<<{[((({<[((()<>)<()[]>){{<>{}}({}())}]{{<[]()>[[]()]}{[()]<{}[]>}}>[[{[<>()]}((<><>><()<>>)](([[]()](()))[
+(([<<{<[<[({{<()<>><[]{}>}{{()()><<><>>}})<[[{(){}}[[][]]][[[]<>]]]{{((){})}[[<>{}]<[][]>]}>][[({({})[[]
+<<((<({[(({[[[{}{}][<>]](<[]{}>[<>{}])]{(([]())(()<>))<[[]()]([]<>)>}}{[(<{}()>((){}))<{{}{}}[()<>]>]({<
+{<{(([(([{{{({<>}({}{}))[[[][]]([]<>)]}<<(()[])[[]]>((<><>)<[][]>)>}}[[[(<[]{}>)]{{([]()){{}<>}>[<[
+([[[{[([{{<{<<()()>(()[])>{({}()){<>[]}}}><(((()())({}[]))([()[]]<<>[]>))[[{{}[]>]{((){})}]>}<[[[[
+{[[[{(<[{(({<<{}()>{<>{}}>[[[]()]<{}{}>]}{[[{}[]]{{}}]((<>))}))[{{{(()[])<{}{}>}<<[][]>{<>()}>}{[{<>{}}<(){}>
+<({[<[(<<{[{<{{}<>}(()())>(<(){}>[<><>])}[([{}]{()[]})<{<>{}}{{}}>]]<{([()[]]<()[]>)[([]())([]<>)]}((((){}
+{[<<[{([((([(({})[{}<>])({[]()}{<>{}})][{{{}<>}}(({}<>))]){<{<<>()>[(){}]}(({}<>)<<>[]>)>[<[()[]]([][])
+<<([<<{[<<<<<(<>{})<{}{}>>>{([()()]<<><>>)(((){}))}>{<<[[]<>][(){}]>[(()<>){[][]}]>{(<()()){<>{}})<[[]{}
+[{<{[{{([<({({(){}}<{}<>>){[{}{}]{()<>}}}[{<{}[]><<>[]>}<[[]()][{}<>]>])<{([{}[]][<>[]])<[<>()](()[])>}[(<
+({([{{<[[[({<(<>())<<>()>>([()[]](<><>))}{<(<>[])<{}()>>({{}[]}{{}<>})})[<{[[][]]({}[])}{{{}[]}}
+{<[([{<[[<([{<[]<>>(()<>)}[(<>{})<{}{}>]][({<><>}<[]()>)(<()()>[{}{}])])>]]<[[({([<><>]{()})<{<>()}{<>
+[(<[[{[{{{<({[()()][[]<>]}<(<>[]){[]{}}>)<[{()()}{[]}]<[{}{}]<{}[]>>>>}{([<[()[]]>{([][]){()[]}}])}}}]<{<[
+{{<{<((<<{[<{[<>()]<<>[]>}<<[]()><{}()>>>({(<>[])<<><>>}{(<>[])(()())})]}>[[[[{<[]()>[()[]]}((<>{})[()<>])
+[<<[<[{(<({{[{[]<>}<{}()>][[<>[]][{}<>]]}<(({}{})({}[]))({[]<>}[{}{}])>}<[(<[]{}>[()()])][[({}[])(()
+<{[{[<<(<{([{({}{}){<>{}}}{[()](()<>)}][[({}())({}<>)][<()<>>{<><>}]])}<({{((){})(()[])}}(<[{}[]]<[][]>><{
+{(<{{[[[<<{{<([]<>)<{}()>)({{}{}}([]{}))}[{({}{})}<[()]>]}{[[<{}<>>](({}{})([][]))]{([{}<>]{()<>
+<([[{{<(<{(<((<>())[[]{}])>((<{}<>>{()<>})[{()()}(()<>)]))}[{(<{{}{}}[{}()]>)}([[[{}[]]<<><>>][<()()>[
+[(<[{<({{<<<<{<>{}}(<><>)>[{[][]}[[][]]]>{([(){}](<><>))((()<>))}><<{{{}<>}{{}<>}}({<>}<[]<>>)>>>{{{(([])[{}{
+((({[[[[<<<{<<[][]><<>()>>[<()[]>(()<>)]}{{<<>{}><<>[]>}{[{}{}][()()]}}><{[[{}[]]<{}[])]{{<>[]}
+[({[[<<<<{[<(<[]<>>[<>()])[[<><>]<<>()>]>>{[{(<><>)[()()]}{(()())[<>{}]}]{{<<><>><[][]>}({<>[]}(()<>
+{[[{(<<([[{(<[{}]>{{<>{}}{{}[]}})}]{<({[(){}][()()]}{(<>{})[[]]})>{<{[[]()][<>[]]}(<[]()>[<>{}]]><{[{}()
+({{[[<<({<<{(<<><>>[{}<>]><((){})>}{{{[]()}([]())}[{{}{}}[<>]]}><<[[{}[]]([][])]<[{}[]]{{}{}}
+<(((<(<<<[[[<{[][]}>{<[]>{<>[]}]]{<<{}[]>[()()]>}]([([()()]<[]()>){{{}()}}]<(({}{})(()()))
+[{{<[{[<<<[[<<{}<>>({}<>)>[<[]()>(()())>]<<<()[]><[][]>><[[]{}]([]{})>>]>><[{(<([])<{}()>>(<{}<>><<>{}>)
+<<<(<[[[<(<<[[[][]][{}{}]]>>(<([[]()))({{}[]}[[][]])>))><[{(<({}<>)(()<>)>{[{}<>]})[({<><>}[<>()])<
+<{<{[(<<([[(<[[]<>][{}[]]>(<()<>>(<>{}>))]]<{{([{}()]{<>()})<(<>()){()}>}{<<()()>(<>[])>{(())[[]{}]}}}(<{[{}
+{[[[[[[{[[{[[[()()][()[]]]{{{}()}(())}]}{<([{}()]{<>()})<[()<>>[<>()]>>[(<[][]>{[]<>})[<{}<>>]]}]]((({({(){}
+(<{(<([([{({{{[][]}{[]<>}}[<{}[]>[<>[]]]})}(<[{{(){}}(<><>)}((()[]))](([{}()]([])){([]<>]{{}()}})>[[[{(
+<{{{<<[<({([[[()[]]]][{(()[])(()())}])}){<{[{<[]()><[][]>}]([{[]()}<{}[]>][([]())[()[]]])}[<[(<>()){
+(<{{[<<[<{[[[(<>[])[<>()]][[[]()][{}()]]][([<>])<[<>[]][[]<>]>]][[({{}<>}[{}{}])({[]()}{()<>})]]}(<([
+{[({([{(((([{<[]()>}<[()()]([])>]{<[()[]]{[]<>}>[<<>{}><()>]}))))<([<[[<()[]><<>()>]][(<()[]><{}<
+({([<[[{([([<{<>()}>[({}[]){<><>}]][((<>){{}{}})(({}())<{}{}>)])])<((({({}{})[<><>]}(((){}))){((
+{[(<[[[{{[{<<{()()}(()())>({()()}[{}{}])><[{(){}}[[][]]]>}<[<(<>{})(<>())>({{}[]}{[]<>})]<{<{}<>>}{<{}<
+<<[({<<{([([{{<>}[<>()]}]{[[[]()]{<>()}](([]{}))})<<[{[]{}}([]())][<[]{}>{[]{}}]>({({}[])}
+<<({[[{{<[<[{[(){}][()[]]}]({{<>}<<>{}>}((<>())<{}()>))>]{{{({[][]}[[]<>])({<><>}({}))}{[{()()}[<>{}]][<<
+{<<[<{<{{({{((()[])[[][]])[{[][]}[<>{}]]}{[{<><>}([][])][[[]]((){})]})){{{([{}{}]((){})){{<><>}
+<[<{{(([(<<[<{{}[]}{[]<>}><([]{})[[]()]>]{{<()<>>([]())}{<()<>><<>[]>}}><[{[<>[]]<[]()>}<<[][]>{[]<>}>][{(()
+<([{{[({[([{(<()()>[<>[]]){<[]{}><[]{}>}}])]<{[[<[{}<>][[]()]>[{{}()}[<>[]]]]{{([]{}]{()<>}}[[()<
+({{{<<<[<<{(<([][])(<>{})>((()[])))}{({<()()>[<>{}]}{{{}[]}(()[])})<[{()())<<>[]>]({<>}<[]<>>)
+<<<{(<({((<({<()>(<><>)})[[<{}()>(<>())]]>(<<({}[])[<>[]])>({{[]()}(<><>)}(([][])<<>()>)))){[[([<>][{}])
+(<((<([({[<<{<{}()>(<><>)}{[[]{}]({}())}>>(<(<<>[]>)<[<>()]<<>{}>]><{<{}<>>}{{<><>}((){})}>)]{[<(<{
+{(([<{(<{<<<(([][])(()[])){<()[]><{}()>}>{(([]<>)<<>()>)<<{}<>>(<>())>}>{(<{{}<>}<[][]>>[[<>{}]<<>{}>])([(<
+{{{{<([([[[[(({}{})<<>>){{<><>}<()[]>}]([[(){}]<{}()>][[<>]<{}{}>])][<([[]<>][{}{}])[<[]<>>[[]<>]]>{<{[
+[<(({{<(({[({[[]<>]<[]<>>}<{{}<>}<(){}>>)[([()()][()[]])<<()()><<>()>>]](<<[{}[]]{[][]}>({[]}[<>{}])>{[[[][]
+(({({{([{{({<<(){}>(()[])}}{{([]<>)[{}[]]}})}<(<{[<><>]}>[[<{}>[{}()]]])<{{{<>()}<<>[]>}}[(<<
+[<((<((<[<{{(<<>[]><()<>>)[<<><>><<><>>]}{([()()][(){}])}}><<<<(<>())[{}]>[([]<>)({}{})]>[[<{
+{<<(({({{{(<{{()[]}}[({})({}())]>)}}}(<[[[[{()<>}<()<>>]([[]{}][<>{}])](<<{}[]>{{}<>}>(<{}<>>{[]{}})>]][({{
+<<{([[[<((<{{(<>[])[()[]]}([[]()])}>))>]]]<<({[(({<{[][]}({}[])>{<{}{}>[<>{}]}}<<[<>()][[]()]>{
+<[[({([<[<{([{[][]}[<><>]]{([]<>)(()<>)})[<(()())<[]<>>>[<[]<>>(()())]]}>(<(<[[]()]<[][]>>{[[]()]{[][]}}
+[<([[({[{{<{{([][])}(<<><>>(()<>))}[[<{}{}>{{}[]}]{{[]{}}}]>}}[<<[{({}{}){{}()}}({[]{}}([]{}))][{[()()]
+((({<(<{(<([<{[]{}}<(){}>><{[][]}(()<>)>](<[[]()]({}[])>{[()<>]}))<{(<[]()>[[][]]>[<[][]>[<>()
+{({<({(<([<([([]{})({}[])])>])<((<[[[]{}]{<>[]}]{{<>[]}<<><>>}>)(<({[]{}}{[]()})[{<>[]}<{}{}>]>{({
+{{{[<<<{<(<{<[[]<>]<[]()>>}<<(()[])([]())><((){})<{}[]>>>><(<((){})<[]>>(([][]))){<[[]{}][()[]
+[<({[(({([<(<[{}<>]>(<()[]>{[]{}})){{{[]()}}[<()<>>[<>[]]]}>]({<<{[]}[<>()]>(<<><>>{[]{}})>[<({}{})[()<>]>[[<
diff --git a/2021/day-10/inputs/sample.txt b/2021/day-10/inputs/sample.txt
new file mode 100644
index 0000000..b1518d9
--- /dev/null
+++ b/2021/day-10/inputs/sample.txt
@@ -0,0 +1,10 @@
+[({(<(())[]>[[{[]{<()<>>
+[(()[<>])]({[<{<<[]>>(
+{([(<{}[<>[]}>{[]{[(<()>
+(((({<>}<{<{<>}{[]{[]{}
+[[<[([]))<([[{}[[()]]]
+[{[{({}]{}}([{[{{{}}([]
+{<[[]]>}<{[{[{[]{()[[[]
+[<(<(<(<{}))><([]([]()
+<{([([[(<>()){}]>(<<{{
+<{([{{}}[<[[[<>{}]]]>[]]
diff --git a/2021/day-10/part_one.ml b/2021/day-10/part_one.ml
new file mode 100644
index 0000000..408fd34
--- /dev/null
+++ b/2021/day-10/part_one.ml
@@ -0,0 +1,29 @@
+open Common
+
+let score ch =
+  match ch with
+  | ')' -> 3
+  | ']' -> 57
+  | '}' -> 1197
+  | '>' -> 25137
+  | _   -> 0
+
+let rec count_errors line stack errors =
+  match (line, stack) with
+  | ([], _) -> errors
+  | (x :: xs, y :: ys) when is_closing x ->
+    if is_pair y x
+    then count_errors xs ys errors
+    else count_errors xs ys (errors + score x)
+  | (x :: xs, _) ->
+    count_errors xs (x :: stack) errors
+
+let () =
+  let path = Sys.argv.(1) in
+
+  read_file path
+  |> List.map (fun cs -> count_errors cs [] 0)
+  |> List.fold_left (+) 0
+  |> print_int;
+
+  print_endline ""
diff --git a/2021/day-10/part_two.ml b/2021/day-10/part_two.ml
new file mode 100644
index 0000000..e83e72f
--- /dev/null
+++ b/2021/day-10/part_two.ml
@@ -0,0 +1,35 @@
+open Common
+
+let score ch =
+  match ch with
+  | '(' -> 1
+  | '[' -> 2
+  | '{' -> 3
+  | '<' -> 4
+  | _   -> 0
+
+let rec score_remaining stack points =
+  match stack with
+  | [] -> points
+  | x :: xs -> score_remaining xs (points * 5 + (score x))
+
+let rec score_closing line stack =
+  match (line, stack) with
+  | ([], _) -> Some (score_remaining stack 0)
+  | (x :: xs, y :: ys) when is_closing x ->
+    if is_pair y x
+    then score_closing xs ys
+    else None
+  | (x :: xs, _) ->
+    score_closing xs (x :: stack)
+
+let () =
+  let path = Sys.argv.(1) in
+
+  read_file path
+  |> List.filter_map (fun cs -> score_closing cs [])
+  |> List.sort compare
+  |> fun lst -> List.nth lst (List.length lst / 2)
+  |> print_int;
+
+  print_endline ""
diff --git a/README.md b/README.md
index 31b39f2..6201c1a 100644
--- a/README.md
+++ b/README.md
@@ -8,4 +8,4 @@
 -   [2018](2018/README.md) (0% completed)
 -   [2019](2019/README.md) (0% completed)
 -   [2020](2020/README.md) (16% completed)
--   [2021](2021/README.md) (34% completed)
+-   [2021](2021/README.md) (38% completed)