text: #include <bits/stdc++.h>
  
  using namespace std;
  using ll = long long;
  
  int main() {
      ios_base::sync_with_stdio(false);
      cin.tie(0); cout.tie(0);
  
      string S;
      cin >> S;
  
      vector<bool> used(26, false);
      for (auto ch : S) {
          int i = (ch - 'a');
          used[i] = true;
      }
      int i;
      for (i = 0; i < 26; ++i) {
          if (used[i]) continue;
          break;
      }
      if (i < 26) {
          cout << char('a' + i) << endl;
      } else {
          cout << "None" << endl;
      }
  
      return 0;
  }