#include <bits/stdc++.h>
using namespace std;
long long int x, y, xx, yy;
int main() {
  cin >> x >> y >> xx >> yy;
  long long int dx = xx - x, dy = yy - y;
  cout << (dy / 2 + 1) * (dx / 2 + 1) + ((dy + 1) / 2) * (dx / 2) << endl;
}
