#include <iostream>
#include <string>
using namespace std;

string s;
bool exist[26];

int main() {
	cin >> s;
	for(int i = 0; i < 26; ++i) {
		exist[i] = false;
	}
	for(int i = 0; i < s.length(); ++i) {
		exist[s[i] - 'a'] = true;
	}
	for(int i = 0; i < 26; ++i) {
		if(exist[i] == false) {
			cout << (char)(i + 'a') << endl;
			break;
		}
		else if(i == 25) {
			cout << "None" << endl;
		}
	}
}