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