"use strict";
const main = input => {
  const alphabets = 'abcdefghijklmnopqrstuvwxyz'.split('');
  let txt = new Set(input.split('').sort());
  let ans = 'None';
  for(let i = 0; i < 26; i++) {
    if(!txt.has(alphabets[i])){
      ans = alphabets[i];
      break;
    }
  }
  console.log(ans);
}


main(require('fs').readFileSync('/dev/stdin', 'utf8'));
