#include<bits/stdc++.h>
// #include<atcoder/all>
using namespace std;
// using namespace atcoder;
/*******  All Required define Pre-Processors and typedef Constants *******/
#define rep(i,n) for(int i=0; i<(n); ++i)
#define rep2(i,a,b) for(int i=a; i<=(b); ++i)
#define rep3(i,a,b) for(int i=a; i>=(b); --i)
#define all(cont) cont.begin(), cont.end()
#define rall(cont) cont.rbegin(), cont.rend()
#define Dcout(a) cout << setprecision(20) << a << endl
#define ll long long
#define ld long double
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vll vector<ll>
#define vpi vector<pii>
#define vpll vector<pll>
#define fi first
#define se second
using Graph = vector<vector<ll>>;
const ll INF64 = (1LL << 60);
const int INF = 1 << 29;
const double pi=acos(-1.0);
/********************************** debug *************************************/
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
/****** Template of some basic operations *****/
struct IoSetup {
    IoSetup() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(10);
        cerr << fixed << setprecision(10);
        }
} iosetup;

template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 >& p) {
    os << p.first << " " << p.second;
    return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
    is >> p.first >> p.second;
    return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
    for(int i = 0; i < (int) v.size(); i++) {
        os << v[i] << (i + 1 != v.size() ? " " : "");
    }
    return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
    for(T &in : v) is >> in;
    return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = ll >
vector< T > make_v(size_t a) {
    return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
    return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
    t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
    for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
    FixPoint(F &&f) : F(forward< F >(f)) {}

    template< typename... Args >
    decltype(auto) operator()(Args &&... args) const {
        return F::operator()(*this, forward< Args >(args)...);
    }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
    return FixPoint< F >{forward< F >(f)};
}
/*********************************************************************/
/*****************++++++*** Start coding ! ***************************/

int main(){
    string s; cin >> s;
    vector<int> a(26,0);
    rep(i,s.size()){
        a[s[i]-'a']++;
    }

    //cout << a << endl;
    bool f = false;
    rep(i,26){
        if(a[i]==0){
            cout << char('a' + i) << endl;
            f = true;
            return 0;
        }
    }

    cout << "None" << endl;
}
/******************************** end ********************************/

Measured memory_bytes: 3784