#include <iostream>
using namespace std;

int A, B, C, D, sum;

int main() {
	cin >> A >> B >> C >> D;
	for (int i = A; i <= B; i++) {
		for (int j = C; j <= D; j++) {
			if (i != j) sum++;
		}
	}
	cout << sum << endl;
	return 0;
}