function Main(input) {
  var s = new Set(input.split("\n")[0]);
  var t = "abcdefghijklmnopqrstuvwxyz"
  for (var i = 0; i < 26; i++) {
    if(!s.has(t[t])) {
      console.log(t[i]);
      return;
    }
  }
  console.log("None");
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
