#include <bits/stdc++.h>
using namespace std;
int main() {
  long int n;
  cin >> n;
  if (n < 6 || (n % 2) != 0) {
    cout << 0;
    exit(0);
  }
  if ((n % 4) == 0)
    cout << (n / 4) - 1;
  else
    cout << n / 4;
  return 0;
}
