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