open Printf
open Scanf

let () =
        let s = read_line() in
        let alpha = "abcdefghijklmnopqrstuvwxyz" in
        let rec iter i =
                (if (i = (String.length alpha)) then
                        "None"
                else if (String.contains s alpha.[i]) then
                        (iter (i + 1))
                else (String.make 1 alpha.[i])
                ) in
        printf "%s\n" (iter 0)
