"use strict";
    
const main = arg => {
    const S  = arg.split("\n")[0].split("");
    const alphabets = "abcdefghijklmnopqrstuvwxyz".split("");

    for(let i in alphabets) {
        if(S.indexOf(alphabets[i]) == -1) {
            console.log(alphabets[i]);
            return;
        }
    }
    
    console.log("None");
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));