diff --git a/bin/day2421.ml b/bin/day2421.ml
new file mode 100644
index 0000000..63f3b0f
--- /dev/null
+++ b/bin/day2421.ml
@@ -0,0 +1,79 @@
+let pos_of_numeric_grid c =
+  match c with
+  | '7' -> (0, 0)
+  | '8' -> (1, 0)
+  | '9' -> (2, 0)
+  | '4' -> (0, 1)
+  | '5' -> (1, 1)
+  | '6' -> (2, 1)
+  | '1' -> (0, 2)
+  | '2' -> (1, 2)
+  | '3' -> (2, 2)
+  | '0' -> (1, 3)
+  | 'A' -> (2, 3)
+  | _ -> raise (invalid_arg "pos_of_numeric_grid")
+
+let pos_of_dir_grid c =
+  match c with
+  | '^' -> (1, 3)
+  | 'A' -> (2, 3)
+  | '<' -> (0, 4)
+  | 'v' -> (1, 4)
+  | '>' -> (2, 4)
+  | _ -> raise (invalid_arg "pos_of_dir_grid")
+
+let invalid_x, invalid_y = (0, 3)
+
+let rec move pos_of_grid builder (cx, cy) code =
+  match Seq.uncons code with
+  | None -> Buffer.contents builder
+  | Some (c, t) ->
+      let dx, dy = pos_of_grid c in
+      if dx > cx && (cy = dy || cy <> invalid_y) then begin
+        Buffer.add_string builder (String.make (dx - cx) '>');
+        move pos_of_grid builder (dx, cy) code
+      end
+      else if dx < cx && (cy = dy || cy <> invalid_y) then begin
+        Buffer.add_string builder (String.make (cx - dx) '<');
+        move pos_of_grid builder (dx, cy) code
+      end
+      else if dy > cy && (cx = dx || cx <> invalid_x) then begin
+        Buffer.add_string builder (String.make (dy - cy) 'v');
+        move pos_of_grid builder (cx, dy) code
+      end
+      else if dy < cy && (cx = dx || cx <> invalid_x) then begin
+        Buffer.add_string builder (String.make (cy - dy) '^');
+        move pos_of_grid builder (cx, dy) code
+      end
+      else begin
+        assert (cx = dx);
+        assert (dy = dy);
+        Buffer.add_char builder 'A';
+        move pos_of_grid builder (cx, cy) t
+      end
+
+let process_code pos_of_grid code =
+  let builder = Buffer.create 30 in
+  move pos_of_grid builder (pos_of_grid 'A') (String.to_seq code)
+
+let human_insns code =
+  let first_robot_insns = process_code pos_of_numeric_grid code in
+  let second_robot_insns = process_code pos_of_dir_grid first_robot_insns in
+  let human = process_code pos_of_dir_grid second_robot_insns in
+  print_endline human;
+  print_endline second_robot_insns;
+  print_endline first_robot_insns;
+  print_endline code;
+  human
+
+let complexity code =
+  let insns = human_insns code in
+  let num = int_of_string (String.sub code 0 (String.length code - 1)) in
+  Printf.printf "%s (%d): %s (%d) (%d) \n" code num insns (String.length insns)
+    (num * String.length insns);
+  num * String.length insns
+
+let part1 codes = List.map complexity codes |> List.fold_left ( + ) 0
+let _ = Aoc.main Aoc.strings_of_file [ (string_of_int, part1) ]
+
+(* 175386 too high *)
diff --git a/bin/dune b/bin/dune
index b8036b3..ee1aa74 100644
--- a/bin/dune
+++ b/bin/dune
@@ -19,7 +19,8 @@
   day2417
   day2418
   day2419
-  day2420)
+  day2420
+  day2421)
  (names
   day2401
   day2402
@@ -40,5 +41,6 @@
   day2417
   day2418
   day2419
-  day2420)
+  day2420
+  day2421)
  (libraries str aoc))