#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x1, x2, y1, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  cout << ((y2 - y1 + 1) >= 0 ? (y2 - y1 + 1) : 0) *
                  (((x2 - x1 + 2) / 2) >= 0 ? ((x2 - x1 + 2) / 2) : 0) -
              ((y2 + 1 - y1) / 2 >= 0 ? (y2 + 1 - y1) / 2 : 0);
}
