#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  while (cin >> s >> t) {
    int n = s.size();
    string res;
    for (int i = n - 1; i >= 0; --i) {
      char& c = s[i];
      ++c;
      int x = 0;
      if (c > 'z') {
        c = 'a';
        x = 1;
      }
      if (!x and s < t) {
        res = s;
        break;
      }
    }
    cout << (res.empty() ? "No such string" : res) << endl;
  }
}
