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