open Printf
open Scanf

let () =
  let s = scanf "%s\n" (fun x -> x) in
  let cs = "abcdefghijklmnopqrstuvwxyz" in
  let rec iter i =
    if i = 26 then "None"
    else
      if String.contains s cs.[i] then iter (i+1)
      else String.make 1 cs.[i]
  in
  printf "%s\n" (iter 0)
