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