function Main(input){
  input=input.split("\n")[0].split("");
  var list ="abcdefghijklmnopqrstuvwxyz".split("");
  for (var i in input){
    var result = list.indexOf(input[i]);
    if(result!=-1)var spliced=list.splice(result,1);
  }

  if(list.length==0){
    console.log("None");
  }else{
    console.log(list[0]);
  }
}

const fs = require("fs");
Main(fs.readFileSync("/dev/stdin","utf-8"));
