open Batteries
let () =
  let s = Scanf.scanf "%s " (fun a -> a) in
  let s_l = String.to_list s in

  let difference xs ys =
    List.fold_left (fun a b -> if List.mem b ys then a else (b::a)) [] xs
  in

  let all = List.init 26 (fun i -> char_of_int (i+97)) in

  Printf.printf "%s\n" @@
  match (difference all s_l) with
    [] -> "None"
  | l -> List.sort compare l |> List.hd |> String.of_char
