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