#include "bits/stdc++.h"
#define rep(i, j) for (int i = 0; i < j; i++)
using namespace std;
using ll = long long;

//const int INF = 1001001001;

/*struct mint {
    ll x; // typedef long long ll;
    mint(ll x = 0) :x((x% mod + mod) % mod) {}
    mint operator-() const { return mint(-x); }
    mint& operator+=(const mint a) {
        if ((x += a.x) >= mod) x -= mod;
        return *this;
    }
    mint& operator-=(const mint a) {
        if ((x += mod - a.x) >= mod) x -= mod;
        return *this;
    }
    mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this; }
    mint operator+(const mint a) const { return mint(*this) += a; }
    mint operator-(const mint a) const { return mint(*this) -= a; }
    mint operator*(const mint a) const { return mint(*this) *= a; }
    mint pow(ll t) const {
        if (!t) return 1;
        mint a = pow(t >> 1);
        a *= a;
        if (t & 1) a *= *this;
        return a;
    }

    // for prime mod
    mint inv() const { return pow(mod - 2); }
    mint& operator/=(const mint a) { return *this *= a.inv(); }
    mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream& operator>>(istream & is, const mint & a) { return is >> a.x; }
ostream& operator<<(ostream & os, const mint & a) { return os << a.x; }*/

int main() {
    string s;
    cin >> s;
    vector<int> abc('z' - 'a' + 1, 0);

    for (int i = 0; i < s.size(); i++) {
        abc[s[i] - 'a']++;
    }
    
    for (int i = 0; i < 26; i++) {
        if (abc[i] == 0) {
            char c;
            switch (i) {
            case 0:
                c = 'a';
                break;
            case 1:
                c = 'b';
                break;
            case 2:
                c = 'c';
                break;
            case 3:
                c = 'd';
                break;
            case 4:
                c = 'e';
                break;
            case 5:
                c = 'f';
                break;
            case 6:
                c = 'g';
                break;
            case 7:
                c = 'h';
                break;
            case 8:
                c = 'i';
                break;
            case 9:
                c = 'j';
                break;
            case 10:
                c = 'k';
                break;
            case 11:
                c = 'l';
                break;
            case 12:
                c = 'm';
                break;
            case 13:
                c = 'n';
                break;
            case 14:
                c = 'o';
                break;
            case 15:
                c = 'p';
                break;
            case 16:
                c = 'q';
                break;
            case 17:
                c = 'r';
                break;
            case 18:
                c = 's';
                break;
            case 19:
                c = 't';
                break;
            case 20:
                c = 'u';
                break;
            case 21:
                c = 'v';
                break;
            case 22:
                c = 'w';
                break;
            case 23:
                c = 'x';
                break;
            case 24:
                c = 'y';
                break;
            case 25:
                c = 'z';
                break;
            }

            cout << c << endl;
            return 0;
        }
    }
    cout << "None" << endl;
    return 0;
}


Measured memory_bytes: 512