#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, st;
  cin >> s >> st;
  int sz = s.size();
  s[sz - 1]++;
  for (int i = sz - 1; i >= 0; i--) {
    if (s[i] == 'z' + 1) {
      if (i == 0)
        cout << "No such string", exit(0);
      else {
        s[i - 1]++;
        s[i] = 'a';
      }
    }
  }
  if (s >= st) cout << "No such string", exit(0);
  cout << s;
  return 0;
}
