#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  int size1 = s.size();
  int size2 = t.size();
  for (int i = size1 - 1; i >= 0; i--) {
    if (s[i] != 'z') {
      s[i]++;
      break;
    } else {
      s[i] = 'a';
    }
  }
  if (t != s) {
    cout << s << endl;
  } else {
    cout << "No such string" << endl;
  }
}
