#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n;
  cin >> n;
  if (n % 2 == 1)
    cout << "0";
  else if (n % 4 == 0)
    cout << n / 4 - 1;
  else
    cout << n / 4;
  return 0;
}
