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