#include <bits/stdc++.h>
using namespace ::std;
int main() {
  long long a, b, c, d, ans;
  cin >> a >> b >> c >> d;
  long long dx = (c - a) >> 1;
  long long dy = (d - b) >> 1;
  cout << dx * dy + (dx + 1) * (dy + 1) << endl;
  return 0;
}
