type 'a seq = ('a -> unit) -> unit

let enum high k =
  for i = 1 to high do k i done
[@@inline always]

let sum s =
  let n = ref 0 in
  s (fun x -> n := !n + x);
  !n
[@@inline always]

let filter pred seq k =
  seq (fun x -> if pred x then k x)
[@@inline always]

let map f seq k =
  seq (fun x -> k (f x))
[@@inline always]

let high = 1000 * 1000 * 100

let res = enum high
	|> filter (fun x -> x mod 2 = 0)
	|> map (( * ) 2)
	|> sum

let _ = Printf.printf "%d\n" res