#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  while (cin >> s >> t) {
    int carry = 1;
    for (int i = s.length() - 1; i >= 0; --i) {
      s[i] += carry;
      if (s[i] > 'z') {
        s[i] -= 26;
        carry = 1;
      } else {
        carry = 0;
      }
    }
    if (s < t) {
      cout << s << endl;
    } else {
      cout << "No such string" << endl;
    }
  }
  return 0;
}
