#include<iostream>
#include<algorithm>
#include <vector>
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++;
			}
		}
	}
	cout << cnt << endl;
}