#include <bits/stdc++.h>
using namespace std;

int main() {
	int A, B, C, D, sum = 0;
	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;
}