#include <iostream>
using namespace std;
int main() {
	int A, B, C, D;
	cin >> A >> B >> C >> D;
	int count = 0;
	for (int i = A; i <= B; i++) {
		for (int j = C; j <= D; j++) {
			if (i != j)count++;
		}
	}
	cout << count << endl;
	return 0;
}