function main(input) {
  var map = {};
  input[0].split("").forEach(w=>{map[w]=true});
  var tx = "abcdefghijklmnopqrstuvwxyz";
  var ans = "None";
  for(var i=0;i<tx.length;i++){
    if(!map[tx[i]]) {console.log(tx[i]); return;}
  }
  console.log(ans);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8').trim().split("\n"))
