let () =
  let s = read_line () in
  try
    Array.init 26 (fun i -> Char.chr @@ Char.code 'a' + i)
    |> Array.to_list
    |> List.find (fun c ->
        Array.fold_left ( && ) true @@ 
        Array.init (String.length s) (fun i -> s.[i] <> c))
    |> Printf.printf "%c\n"
  with Not_found -> print_endline "None"
