let () =
  let n = 13 + (Random.int 23) in
  let _n = ref n in
  let run = ref true in
  while !run do
    Printf.printf "%d\n" !_n;
    let line = read_line () in
    begin
      try
        let d = int_of_string line in
        _n := !_n - d ;
      with _ ->
        Printf.printf "please input an integer number\n%!";
    end;
    if !_n <= 1 then run := false
  done