#include <algorithm>
#include <cassert>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
typedef long long ll;

#define rep(i, n) for(ll i = 0; i < n; i++)
#define REP(i, n) for(ll i = 1; i < n + 1; i++)
#define PI 3.14159265359
#define EPS 0.0000000001
#define MOD 1000000007
//cout << std::fixed << std::setprecision(15) << y << endl;


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

    sort(S.begin(), S.end());

    char ans = 'a';
    rep(i, S.size()){
        if(S[i] == ans){
            ans++;
        }else if(S[i] > ans){
            break;
        }
    }

    if(ans > 'z'){
        cout << "None" << endl;
    }else{
        cout << ans << endl;
    }
    
    return 0;
}
#include "bits/stdc++.h"
 
using namespace std;
typedef long long ll;
typedef vector<ll> vll;
#define int ll
typedef pair<int,int> pii;
#define fi first
#define se second
#define Sort(a) sort(a.begin(),a.end())
#define rep(i,n) for(int i = 0;i < (n) ; i++) 
#define REP(i,n) for(int i = 0;i < (n) ; i++) 
#define MP(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define INF LLONG_MAX/4
#define INTINF INT_MAX/2
#define all(x) (x).begin(),(x).end()
#define debug(x) cerr<<#x<<": "<<x<<endl
#define debug_vec(v) cerr<<#v<<":";rep(i,v.size())cerr<<" "<<v[i];cerr<<endl
template<class T> inline bool chmax(T &a, T b){if(a < b){a = b;return true;}return false;}
template<class T> inline bool chmin(T &a, T b){if(a > b){a = b;return true;}return false;}
/*----------------------------------------------------------------*/
const int MOD = 998244353;
// const int MOD = 1000000007;

/*----------------------------------------------------------------*/
signed main(){

	
	string s;
	cin >> s;
	int cnt[26] = {};
	for(auto c:s)cnt[c-'a']++;

	rep(i,26){
		if(cnt[i] == 0){
			cout << (char)('a'+i) << endl;
			return 0;
		}
	}
	cout << "None" << endl;

	
	return 0;
}
/*----------------------------------------------------------------*/

// g++ -std=gnu++17 code1.cpp
// sudo pip3 install --upgrade online-judge-tools
// rm -r -f test;oj dl https://code-festival-2018-quala.contest.atcoder.jp/tasks/code_festival_2018_quala_c
// rm -r -f test;oj dl https://abc071.contest.atcoder.jp/tasks/abc071_b