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