#include <iostream>
#include <algorithm>
using namespace std;

int a, b, c, d;

int main() {
	cin >> a >> b >> c >> d;
	if (a > c) {
		swap(a, c);
		swap(b, d);
	}
	int kasanari = max(b - c + 1, 0);
	
	cout << (b - a + 1) * (d - c + 1) - kasanari << endl;
	return 0;
}