'use strict'

function main(s) {
  s = s.split('\n')[0].split('');

  const str = new Set(s);
  const abc = [...Array(26)].map((c, i) => {
    return String.fromCharCode('a'.charCodeAt(0) + i);
  });
  
  let ans = '';
  for (const char of abc) {
    if (!(str.has(char))) {
      ans = char;
      break;
    }
  }

  console.log(ans ? ans : 'None');
}

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