function main(input) {
  input=input.trim();
  var s=input
  var all=["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"]
  for(var i=0; i<s.length; i++){
    var temp=s.substr(i,1)
    //console.log(all.indexOf(temp));
    all.splice(all.indexOf(temp),1)
    //console.log(all);
  }
  //console.log(all);
  console.log(all[0]);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));