#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, c, d;
  cin >> a >> b >> c >> d;
  long long ac = abs(a - c), bd = abs(b - d);
  ac /= 2;
  bd /= 2;
  long long ans = ac * bd + (ac + 1) * (bd + 1);
  cout << ans;
  return 0;
}
