#include <bits/stdc++.h>
using namespace std;
int main() {
  std::ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    string s, t;
    cin >> s >> t;
    int n = s.length();
    int add = 1;
    int i = n - 1;
    while (add) {
      if (s[i] < 'z') {
        s[i]++;
        add = 0;
      } else {
        s[i] = 'a';
      }
      i--;
    }
    if (s < t) {
      cout << s << "\n";
    } else {
      cout << "No such string"
           << "\n";
    }
  }
}
