#include <bits/stdc++.h>
using namespace std;
 // cout << (char)(z + 1);

int main(){
  string S;
  int frag;
  cin >> S;
  for (char c = 'a'; c != '{'; c = (char)(c + 1)){
    frag = 0;
    for (int i = 0; i < S.size(); i++){
      if(S.at(i) == c){
        frag = 1;
      }
    }
    if(frag == 0){
      cout << c << endl;
      return 0;
    }
  }
  cout << "None" << endl;
  return 0;
}
#include<iostream>
#include<string>
#include<iterator>

std::string reverse_str(std::string str);

int main()
{
	std::string str;
	while (std::cin >> str)
	{
		str = reverse_str(str);
		std::cout << str << std::endl;
	}
	return 0;
}

std::string reverse_str(std::string str)
{
	std::string str_ans;
	for (unsigned long i = 1; i <= str.size(); ++i)
	{
		std::string::iterator it = std::prev(end(str), i);
		str_ans += *it;
	}
	return str_ans;
}