Scanf.scanf "%s" (fun s ->
    let n = String.length s in
    let rec loop i =
        if i = n then i - 1 else
            if s.[i] = '3' then loop (i + 1)
            else (-1)
    in
    (if s.[0] <> '1' then (-1) else loop 1) |> Printf.printf "%d\n"
)