#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, t;
  cin >> s >> t;
  string temp = s;
  int x = temp.size() - 1;
  while (temp[x] == 'z' && x != 0) temp[x] = 'a', x--;
  if (x == -1)
    cout << "No such string\n";
  else {
    temp[x]++;
    if (temp == t) {
      cout << "No such string\n";
    } else
      cout << temp << "\n";
  }
}
