#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s, t;
  cin >> s >> t;
  for (int i = (int)s.size() - 1; i >= 0; --i) {
    if (isalpha(s[i] + 1)) {
      ++s[i];
      if (s < t) {
        cout << s;
        return 0;
      } else
        break;
    } else
      s[i] = 'a';
  }
  cout << "No such string";
  return 0;
}
