#include <bits/stdc++.h>
using namespace std;

int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;

	int cnt = 0;
	for (int i = a; i <= b; i++) {
		for (int j = c; j <= d; j++) {
			if (i == j) {
				cnt++;
			}
		}
	}

	int s = (b - a + 1) * (d - c + 1);
	cout << s - cnt << endl;
}