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