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