#include <bits/stdc++.h>
using namespace std;
string s, t;
void next(int i) {
  if (i >= 0) {
    if (s[i] != 'z')
      s[i] += 1;
    else {
      s[i] = 'a';
      next(--i);
    }
  }
}
int main() {
  cin >> s >> t;
  if (s != t) {
    int si = ((int)s.size());
    next(si - 1);
    if (s == t)
      cout << "No such string" << endl;
    else
      cout << s << endl;
  } else
    cout << "No such string" << endl;
  return 0;
}
