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