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