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