#include <bits/stdc++.h>
using namespace std;
string s, t;
int main() {
  cin >> s;
  cin >> t;
  s[s.size() - 1]++;
  reverse(s.begin(), s.end());
  for (int i = 0; i < s.size(); i++) {
    if (s[i] > 'z') {
      s[i] -= 26;
      if (i + 1 < s.size())
        s[i + 1]++;
      else
        s += 'a';
    }
  }
  reverse(s.begin(), s.end());
  if (s < t)
    cout << s << endl;
  else
    cout << "No such string" << endl;
}
