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