#include <bits/stdc++.h>
using namespace std;

template<typename T> istream& operator>>(istream &s, vector<T> &v) {
    for (T &t : v) s >> t;
    return s;
}
template<typename T> ostream& operator<<(ostream &s, const vector<T> &v) {
    for (const T &t : v) s << t << endl;
    return s;
}
template<typename T> T min(const vector<T>& v) {return *min_element(v.begin(), v.end());}
template<typename T> T max(const vector<T>& v) {return *max_element(v.begin(), v.end());}
template<typename T> int min_element(vector<T>& v) {return min_element(v.begin(), v.end()) - v.begin();}
template<typename T> int max_element(vector<T>& v) {return max_element(v.begin(), v.end()) - v.begin();}
template<typename T> void sort(vector<T>& v) {sort(v.begin(), v.end());}
template<typename T> void greatersort(vector<T>& v) {sort(v.begin(), v.end(), greater<>());}
template<typename T, typename Function> void sort(vector<T>& v, Function func) {sort(v.begin(), v.end(), func);}
template<typename T> void rsort(vector<T>& v) {sort(v.rbegin(), v.rend());}
template<typename T> void reverse(vector<T>& v) {reverse(v.begin(), v.end());}
template<typename T> void unique(vector<T>& v) {v.erase(unique(v.begin(), v.end()), v.end());}
template<typename T> void nth_element(vector<T>& v, int n) {nth_element(v.begin(), v.begin() + n, v.end());}
template<typename T> bool next_permutation(vector<T>& v) {return next_permutation(v.begin(), v.end());}
template<typename T> int find(vector<T>& v, T t) {return find(v.begin(), v.end(), t) - v.begin();}
template<typename T> int in(vector<T> v, T t) {return find(v, t) != int(v.size());}
template<typename T> int lower_bound(vector<T>& v, T t) {return lower_bound(v.begin(), v.end(), t) - v.begin();}
template<typename T> int upper_bound(vector<T>& v, T t) {return upper_bound(v.begin(), v.end(), t) - v.begin();}
template<typename T> T accumulate(const vector<T>& v, function<T(T, T)> func = plus<T>()) {return accumulate(v.begin(), v.end(), T(), func);}
template<typename T> void adjacent_difference(vector<T>& v) {adjacent_difference(v.begin(), v.end(), v.begin());}
template<typename T> void adjacent_difference(vector<T>& v, vector<T>& u) {adjacent_difference(v.begin(), v.end(), u.begin());}
template<typename T> vector<T> partial_sum(const vector<T>& v) {
    vector<T> u(v.size());
    partial_sum(v.begin(), v.end(), u.begin());
    return u;
}\
template<typename T> T inner_product(vector<T>& v, vector<T>& u) {return inner_product(v.begin(), v.end(), u.begin(), T(0));}
template<typename T> int count(const vector<T>& v, T t) {return count(v.begin(), v.end(), t);}
template<typename T, typename Function> int count_if(const vector<T>& v, Function func) {return count_if(v.begin(), v.end(), func);}
template<typename T, typename Function> void remove_if(vector<T>& v, Function func) {v.erase(remove_if(v.begin(), v.end(), func), v.end());}
template<typename T, typename Function> bool all_of(vector<T> v, Function func) {return all_of(v.begin(), v.end(), func);}
template<typename T, typename Function> bool any_of(vector<T> v, Function func) {return any_of(v.begin(), v.end(), func);}
template<typename T, typename Function> bool none_of(vector<T> v, Function func) {return none_of(v.begin(), v.end(), func);}
template<typename T> vector<T> subvector(vector<T>& v, int a, int b) {return vector<T>(v.begin() + a, v.begin() + b);}
template<typename T> int kinds(const vector<T>& v) {return set<T>(v.begin(), v.end()).size();}
template<typename T> map<T, int> count_kinds(const vector<T>& v) {
    map<T, int> res;
    for (const auto& i : v) ++res[i];
    return res;
}
template<typename T> void iota(vector<T>& v, T t = 0) {iota(v.begin(), v.end(), t);}
template<typename T> bool is_sorted(const vector<T>& v) {return is_sorted(v.begin(), v.end());}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    vector<bool> check(200, false);
    string S;
    cin >> S;
    for (int i = 0; i < S.length(); ++i) {
        check[S[i]] = true;
    }
    for (char i = 'a'; i <= 'z'; ++i) {
        if (!check[i]) {
            cout << i << endl;
            return 0;
        }
    }
    cout << "None" << endl;
    return 0;
}



Measured memory_bytes: 512