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