#include <bits/stdc++.h>
using namespace std;
int main() {
  int i = 0;
  string s, s2;
  cin >> s >> s2;
  for (int i = s.length() - 1; i >= 0; i--) {
    if (s[i] == 'z') {
      s[i] = 'a';
    } else {
      s[i]++;
      break;
    }
  }
  if (s < s2) {
    cout << s;
  } else {
    cout << "No such string";
  }
  return 0;
}
