#include<bits/stdc++.h>
using namespace std;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
 
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep1(i, n) for (ll i = 1; i <= (ll)(n); i++)
#define INF 10000000000
#define MOD 1000000007
using ll = long long;
using pint = pair<int,int>;
using pll = pair<ll,ll>;
using Graph = vector<vector<int>>;

int main(){
    set<char> m;
    string s; cin>>s;
    for(auto c:s)m.insert(c);
    for(char i = 'a'; i <= 'z'; i++){
        if(!m.count(i)){
            cout<<i<<endl;
            return 0;
        }
    }
    cout<<"None"<<endl;
}
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 

using namespace __gnu_pbds; 
using namespace std;
using   ll=long long;
using   ld=long double;

#define fast        	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define ordered_set 	tree<pll, null_type,less<pll>, rb_tree_tag,tree_order_statistics_node_update>

#define endl        	"\n"
#define pb          	push_back
#define mp          	make_pair
#define F           	first
#define S           	second
#define int             long long int
#define pll				pair<int , int>

#define ALL(v)      	v.begin(),v.end()
#define ALLR(v)     	v.rbegin(),v.rend()
#define pii         	3.14159265358979323
#define inf     		LLONG_MAX
#define ones(x)     	__builtin_popcount(x)
#define fill(a,b)   	memset(a,b,sizeof(a))
#define mod 			1000000007
#define hell            998244353 

ll mod_pow(ll a,ll b,ll m) 
{
    ll res = 1;
    while(b)	
    {
        if(b&1) 	
        {
        	res=(res*a) % m;
        }
        a=(a*a) % m;
        b>>=1;
    }
    return res;
}

ll mod_inverse(ll a) 
{
	return mod_pow(a , mod-2 , mod);
}

void solve()
{
	string s;
	
	cin >> s;
	
	int f[26] {};
	
	for(auto u : s) {
		f[u-'a']++;
	}
	
	for(int i = 0; i < 26; ++i) {
		if(f[i] == 0) {
			cout << (char)(i+'a');
			
			return;
		}
	}
	
	cout << "None";
}

signed main() {
    fast;
    
    int t = 1;
    
	//cin >> t;
    
    while(t--) {
        solve();
    }
 
    return 0;
}
