#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  cin >> a >> b;
  string c = a;
  int n(a.length());
  for (int i = n - 1; i >= 0; i--) {
    while (c[i] < 'z') {
      c[i]++;
      if (c < b) {
        cout << c << endl;
        return 0;
      }
    }
    if (c[i] == 'z') c[i] = 'a';
  }
  cout << "No such string" << endl;
  return 0;
}
