process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = []
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', (line) => {
  lines.push(line);
});

reader.on('close', () => {
 var s = lines[0].split('');
 var sset = new Set(s);
 var sarray = Array.from(sset);
 sarray.sort();
 var alpha = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'];

 var ans = alpha.filter(function(value){
   return sarray.indexOf(value) == -1;
 });
 if(ans[0] == undefined){
   console.log('None');
 }else{
   console.log(ans[0]);
 }
});