function Main(input) {
  var s = input.replace(/\r?\n/g, "");
  var a = "abcdefghijklmnopqrstuvwxyz"
  for (var i of a) {
    if(s.indexOf(i) == -1) {
      console.log(i);
      return;
    }
  }
  console.log("None");
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));