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