#include <bits/stdc++.h>
using namespace std;
int main() {
    string s;
    int a[26];
    for (int i = 0; i < 26; i++) a[i] = 0;
    bool occur = true;
    cin >> s;
    for (int i = 0; i < s.size(); i++)
    {
        a[s[i] - 97]++;
    }
    for (int i = 0; i < 26; i++)
    {
        if (a[i] == 0)
        {
            occur = false;
            cout << char(i + 97);
            break;
        }
    }
    if (occur == true) cout << "None";
}
#include <bits/stdc++.h>
#define PI 3.14159265359
#define NIL -1
using namespace std;
const int64_t MOD = 1e9 + 7;

int main() {
	string S;
	cin >> S;

	vector<bool> v(26);
	for (int i = 0; i < S.size(); i++) {
		char ch = S.at(i);
		int n = (int)ch - 97;
		v.at(n) = 1;
	}

	string ans = "None";
	for (int i = 0; i < 26; i++) {
		if (v.at(i) == 0) {
			ans = (char)i + 97;
			break;
		}
	}

	cout << ans << endl;
}