let rec exist lst = match lst with
  | [] -> true
  | h :: t -> if h <> 3 then false else exist t
let () =
  let n = Scanf.scanf "%d\n" (fun x -> x) in
  let list_of_string str =
    let l = String.length str in
    let rec iter i lst =
      if i >= l then lst
      else iter (i+1) ((int_of_char str.[i] - 48) :: lst) in
    iter 0 [] in
  let slst = List.rev (list_of_string (string_of_int n)) in
  let head = List.hd slst in
  let tail = List.tl slst in
  let n = if head = 1 && (exist tail) then List.length tail else -1 in
  Printf.printf "%d\n" n