#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < n; i++)
#define vv(T) vector<vector<T>>
#define coa cout << ans << endl; return 0;
using namespace std;
using ll = long long;
template<typename T>
using vec = vector<T>;
template<typename T>
using vvec = vector<vector<T>>;
using qint = queue<int>;
using sint = stack<int>;
using pii = pair<int, int>;


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

    map<int, int> alphabet;
    for(auto c : s){
        alphabet[c]++;
    }
    
    string ans;
    for(char c = 'a'; c <= 'z'; c++){
        if(alphabet[c] == 0){
            ans.push_back(c);
        }
    }

    if(ans.empty()) ans += "None";
    else ans = ans[0];

    coa
}

#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x ,n) for(int i = x; i < n; i++)
#define repr(i, n) for(int i = n; i >= 0; i--)
#define INF 2e9
#define ALL(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
int main(){
    string az="abcdefghijklmnopqrstuvwxyz";
    string s;
    cin >> s;
    sort(s.begin(),s.end());
    
    int i=0,j=0;
    rep(j,s.size()){
        if(az[i]==s[j])i++;
        if(i==26){cout << "None" << endl;return 0;}
    }
    
    cout << az[i];
    
	return 0;
}