diff --git a/day21.erl b/day21.erl
new file mode 100644
index 0000000..9941fc8
--- /dev/null
+++ b/day21.erl
@@ -0,0 +1,42 @@
+-module(day21).
+
+-export([solve/1]).
+
+solve(InputData) ->
+    {eval(<<"root">>, parse_input(InputData)), none}.
+
+parse_input(Input) -> parse_input(Input, #{}).
+
+parse_input(<<>>, Acc) -> Acc;
+parse_input(<<$\n, Rest/binary>>, Acc) -> parse_input(Rest, Acc);
+parse_input(<<Name:4/bytes, $:, $\s, Left:4/bytes, $\s, Op, $\s, Right:4/bytes, Rest/binary>>, Monkeys) ->
+    parse_input(Rest, Monkeys#{Name => {Op, Left, Right}});
+parse_input(<<Name:4/bytes, $:, $\s, RestIn/binary>>, Monkeys) ->
+    {Value, RestOut} = parse_int(RestIn),
+    parse_input(RestOut, Monkeys#{Name => Value}).
+
+eval(Name, Monkeys) ->
+    #{Name := Node} = Monkeys,
+    case Node of
+        Value when is_integer(Value) -> Value;
+        {Op, Left, Right} ->
+            LeftValue = eval(Left, Monkeys),
+            RightValue = eval(Right, Monkeys),
+            case Op of
+                $+ -> LeftValue + RightValue;
+                $- -> LeftValue - RightValue;
+                $/ -> LeftValue div RightValue;
+                $* -> LeftValue * RightValue
+            end
+    end.
+
+%% Parse Integer standard code
+parse_int(<<$-, Rest/binary>>) ->
+    {AbsoluteValue, Binary} = parse_int0(Rest, 0),
+    {-1 * AbsoluteValue, Binary};
+parse_int(Binary) ->
+    parse_int0(Binary, 0).
+
+parse_int0(<<Digit, Rest/binary>>, Acc) when Digit >= $0 andalso Digit =< $9 ->
+    parse_int0(Rest, Acc*10 + Digit - $0);
+parse_int0(Binary, Acc) when is_binary(Binary) -> {Acc, Binary}.