(* O(|s|) *)
let s = read_line () in
let flags = Array.make 128 false in
String.iter (fun c -> flags.(int_of_char c) <- true) s;
for i = int_of_char 'a' to int_of_char 'z' do
  if not flags.(i) then (Printf.printf "%c" @@ char_of_int i; exit 0)
done;
print_endline "None"