#include<iostream>
#include<string>
using namespace std;
int main(){

  string S;
  cin>>S;
  char check='a';
  bool flag=0;
  for(check='a';check<='z';check++){
  
    flag=1;
    for(size_t i=0;i<=S.length()-1;i++){
    
      if(S[i]==check){
      
        flag=0;
        break;
      }
    }
    if(flag==1){
    
      cout<<check;
      return 0;
    }
  }
  cout<<"None";
  return 0;
}
/*
ID: anonymo14
TASK: wormhole
LANG: C++                 
*/
#include<bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pair<int,int>> vpii;

#define F first
#define S second
#define PU push
#define PUF push_front
#define PUB push_back
#define PO pop
#define POF pop_front
#define POB pop_back

#define REP(i,a,b) for(int i=a; i<=b; i++)

#define MOD 1000000007

void solve(int test_case) {
	string s;
	cin>>s;
	bitset<26> b;
	for(char c:s)b[(int)(c-'a')]=1;
	int i = 0;
	for(;i<26;i++) {
		if(!b[i]){break;}
	}
	if(i<26)cout<<(char)(i+'a'); else cout<<"None";
}

int main() {
	
	////// FILE BASED IO////
	//freopen("wormhole.in", "r", stdin);
	//freopen("wormhole.out", "w", stdout);
	///////////////
	
	ios::sync_with_stdio(0);
	cin.tie(0);
	int t=1;
	//cin>>t;
	REP(i,1,t) {
		solve(i);
	}
	return 0;
}	
