#include <bits/stdc++.h>
using namespace std;
string s, t;
void increase() {
  int i;
  for (i = ((int)s.size()) - 1; i >= 0; --i)
    if (s[i] + 1 <= 122) {
      ++s[i];
      break;
    } else
      s[i] = 'a';
  if (i < 0 || s == t)
    cout << "No such string";
  else
    cout << s;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> s >> t;
  int i;
  for (i = 0; i < ((int)s.size()); ++i)
    if (s[i] != t[i]) break;
  if (s[i] - t[i] == -1 && i == ((int)s.size()) - 1)
    cout << "No such string";
  else
    increase();
}
