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