#include <bits/stdc++.h>
using namespace std;
string s1, s2;
string ans;
int main() {
  cin >> s1 >> s2;
  int pos = s1.size() - 1;
  string s = s1;
  while (s1[pos] == 'z') s1[pos] = 'a', pos--;
  s1[pos]++;
  if (s1 == s2)
    cout << "No such string";
  else
    cout << s1;
  return 0;
}
