open Printf
open Scanf

let id x = x

let s = read_line ()

let () =
    let ary = Array.init 26 (fun i -> 0) in
    String.iter (fun c ->
        let idx = int_of_char c - int_of_char 'a' in
        ary.(idx) <- ary.(idx) + 1
    ) s;
    try
        for i = 0 to 25 do
            if ary.(i) = 0 then begin
                printf "%c\n" (char_of_int (i + int_of_char 'a'));
                failwith ""
            end
        done;
        print_endline "None"
    with Failure _ -> ()
