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