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