'use strict'
 
function main(n) {
  let N = new Set(n.split("\n")[0].split("").sort());
  const alphabets = 'abcdefghijklmnopqrstuvwxyz'.split('');
  let ans = "None";
  rooptag:
  for(let n = 0; n < N.size; n++ ){
      if(!N.has(alphabets[m])){
      	ans = alphabets[m];
        break rooptag;
      }
    }
  }
  
  console.log(ans);
}

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