#include <bits/stdc++.h>
using namespace std;
int main() {
  string min_s, max_s;
  cin >> min_s >> max_s;
  if (min_s == max_s || min_s > max_s)
    cout << "No such string";
  else {
    for (int i = max_s.size() - 1; i >= 0; i--)
      if (max_s[i] > 'a') {
        max_s[i]--;
        for (int j = i + 1; j < max_s.size(); j++) max_s[j] = 'z';
        break;
      }
    cout << (max_s > min_s ? max_s : "No such string");
  }
  return 0;
}
