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