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