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

#define ll long long
#define mod 1000000007
#define mod2 998244353
#define pii pair<ll,ll>
#define pb push_back

#define debug(a) { cout << #a << " = " << a << endl;}

void sep() {
    cout << "-------------BLOCK-----------------\n";
}

template<typename T>
void debugv(vector<T> arr) {
    for (auto t: arr)
        cout << t << " ";
    cout << "\n";
}

template<typename T>
void debugarr(T arr, int n) {
    for (int i=0;i<n;i++) cout << arr[i] << " ";
    cout << "\n";
}

ll __min(ll a, ll b) {
    return a < b ? a : b;
}

ll __max(ll a, ll b) {
    return a > b ? a : b;
}

const int maxn = 1e5+1;
ll arr[maxn], n, m, t, k, q;
unordered_map<ll,ll> u;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    
    int arr[26]={0};
    string s;
    cin>>s;
    for (char c: s) arr[c-'a']++;
    for (int i=0;i<26;i++) {
        if (arr[i] == 0) {
            char c = 'a' + i;
            cout << c << endl;
            exit(0);
        }
    }
    cout << "None\n";
}

#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;

int main() {
  string s;
  cin >> s;
  vector<bool> found(26, false);
  rep(i, s.size()) found[s[i] - 'a'] = true;
  int a = -1;
  rep(i, 26) if (found[i] == false) {
    a = i;
    break;
  }
  if (a == -1) cout << "None" << endl;
  else {
    char c = a + 'a';
    cout << c << endl;
  }
  return 0;
}