#include <iostream>
using namespace std;
int main(void) {
	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;
		}
	}
	cout << cnt << endl;
	return 0;
}