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