    #include <bits/stdc++.h>
    using namespace std;
    #define all(x) (x).begin(), (x).end()
    #define SZ(X) ((int)(X).size())
    #define endl "\n";
    int main()
    {
    	ios::sync_with_stdio(0);
    	cin.tie(0);
    	// freopen("input.txt", "r", stdin);
    	// freopen("output.txt", "w", stdout);
    	string s,a;
    	cin>>s;
    	set<char> t(all(s));
    	a = "abcdefghijklmnopqrstuvwxyz";
    	
    	for (auto c:a)
    	{
    		auto pos =t.find(c);
    		if(pos==t.end()){
    			cout<<c;	
    			return 0;
    		}
    		
    	}
    cout<<"None";
     
       return 0;
    }
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<utility>
#include<tuple>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>

using namespace std;

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define BIL ((ll)1e9)
#define MOD ((ll)1e9+7)
#define INF (1LL<<60)           //1LL<<63でオーバーフロー
#define inf (1<<29)             //1<<29でオーバーフロー

int main(int argc,char* argv[]){
    string s;
    cin >> s;

    set<char> c;
    for(int i=0;i<26;i++){
        c.insert((char)('a'+i));
    }

    bool ans=true;
    for(int i=0;i<s.size();i++){
        if(c.count(s[i]))c.erase(s[i]);
        if(c.empty())ans=false;
    }

    if(ans){
        auto it = c.begin();
        cout << *it << endl;
    }
    else cout << "None" << endl;

    return 0;
}
