let () =
  let a = Array.make 30 false in
  let rec read = function
      0 -> ()
    | i ->
       let n = read_int () in
       a.(n-1) <- true;
       read (i-1)
  in
  let () = read 28 in
  Array.iteri (fun i x -> if not x then Printf.printf "%d\n" (i+1)) a
;;