diff --git a/bin/day2420.ml b/bin/day2420.ml
new file mode 100644
index 0000000..b4b6a98
--- /dev/null
+++ b/bin/day2420.ml
@@ -0,0 +1,87 @@
+(** [dijkstra visit check_end states] executes Dijkstra's algorithm.
+
+    [visit cost state] is called to visit [state] with [cost]. It should mark
+    [state] as visited, and return a list of [(cost, state)] pairs which contain
+    new states to examine. The returned list should be sorted by [cost].
+
+    [check_end state] should return [true] if and only if [state] is an end
+    state.
+
+    [states] is a list of [(cost, state)] pairs ordered by [cost].
+
+    [dijkstra] returns [None] if no path is found to the destination. It returns
+    [Some (cost, state, remaining_states)] if a route is found. [cost] is the
+    cost of getting to [state]. [remaining_states] is a list of the remaining
+    states which can be passed back to [dijkstra] if we want to find further
+    paths. *)
+let rec dijkstra visit check_end =
+  let compare_costs (lhs, _) (rhs, _) = compare lhs rhs in
+  function
+  | [] -> None
+  | (cost, state) :: t ->
+      if check_end state then Some (cost, state)
+      else
+        let new_states = visit cost state |> List.merge compare_costs t in
+        dijkstra visit check_end new_states
+
+let visit grid has_visited cost ((x, y) as state) =
+  if not (Aoc.Grid.pos_is_valid grid state) then []
+  else if has_visited.(Aoc.Grid.idx_of_pos grid state) then []
+  else begin
+    has_visited.(Aoc.Grid.idx_of_pos grid state) <- true;
+    [
+      (cost + 1, (x - 1, y));
+      (cost + 1, (x + 1, y));
+      (cost + 1, (x, y - 1));
+      (cost + 1, (x, y + 1));
+    ]
+  end
+
+let find_cost grid start =
+  let has_visited =
+    Array.init (Aoc.Grid.length grid) (fun x ->
+        Aoc.Grid.get_by_idx grid x = '#')
+  in
+  match
+    dijkstra (visit grid has_visited)
+      (fun x -> Aoc.Grid.get_by_pos_opt grid x = Some 'E')
+      [ (0, start) ]
+  with
+  | None -> failwith "find_cost"
+  | Some (cost, _) -> cost
+
+module IntMap = Map.Make (Int)
+
+let update_value = function None -> Some 1 | Some x -> Some (x + 1)
+
+let find_cost_reductions grid start =
+  let max_cost = find_cost grid start in
+  let rec impl acc idx =
+    if idx >= Aoc.Grid.length grid then acc
+    else if Aoc.Grid.get_by_idx grid idx <> '#' then impl acc (idx + 1)
+    else
+      let grid' = Aoc.Grid.update_idx grid idx '.' in
+      let new_cost = find_cost grid' start in
+      impl (IntMap.update (max_cost - new_cost) update_value acc) (idx + 1)
+  in
+  impl IntMap.empty 0
+
+let print_map map = IntMap.iter (fun k v -> Printf.printf "%d -> %d\n" k v) map
+
+let part1 (grid, start) =
+  let map = find_cost_reductions grid start in
+  print_map map;
+  let map = IntMap.filter (fun k _ -> k >= 100) map in
+  IntMap.fold (fun _ v a -> v + a) map 0
+
+let find_start grid =
+  match Aoc.Grid.idx_from_opt grid 0 'S' with
+  | None -> failwith "find_start"
+  | Some x -> Aoc.Grid.pos_of_idx grid x
+
+let data_of_file fname =
+  let grid = Aoc.Grid.of_file fname in
+  let start = find_start grid in
+  (grid, start)
+
+let _ = Aoc.main data_of_file [ (string_of_int, part1) ]
diff --git a/bin/dune b/bin/dune
index c24ba0f..b8036b3 100644
--- a/bin/dune
+++ b/bin/dune
@@ -18,7 +18,8 @@
   day2416
   day2417
   day2418
-  day2419)
+  day2419
+  day2420)
  (names
   day2401
   day2402
@@ -38,5 +39,6 @@
   day2416
   day2417
   day2418
-  day2419)
+  day2419
+  day2420)
  (libraries str aoc))
diff --git a/lib/aoc.ml b/lib/aoc.ml
index 62a3885..35aa405 100644
--- a/lib/aoc.ml
+++ b/lib/aoc.ml
@@ -66,14 +66,15 @@ module Grid = struct
 
   let idx_from_opt grid = String.index_from_opt grid.grid
 
-  let update_pos grid pos c =
-    let idx = idx_of_pos grid pos in
+  let update_idx grid idx c =
     let builder = Buffer.create (length grid) in
     Buffer.add_string builder (String.sub grid.grid 0 idx);
     Buffer.add_char builder c;
     Buffer.add_string builder
       (String.sub grid.grid (idx + 1) (length grid - idx - 1));
     { grid with grid = Buffer.contents builder }
+
+  let update_pos grid pos c = update_idx grid (idx_of_pos grid pos) c
 end
 
 let log10i i =
diff --git a/lib/aoc.mli b/lib/aoc.mli
index aa194ee..c9682a8 100644
--- a/lib/aoc.mli
+++ b/lib/aoc.mli
@@ -143,4 +143,8 @@ module Grid : sig
   val update_pos : t -> int * int -> char -> t
   (** [Grid.update_pos grid pos c] returns a grid with the character at position
       [pos] changed to [c]. *)
+
+  val update_idx : t -> int -> char -> t
+  (** [Grid.update_pos grid idx c] returns a grid with the character at index
+      [idx] changed to [c]. *)
 end