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

int main() {
	int A, B, C, D, ans;
	cin >> A >> B >> C >> D;
	ans = (B - A + 1) * (D - C + 1);
	for (int i = 1; i <= 100; i++) {
		if (A <= i && i <= B && C <= i && i <= D) ans--;
	}
	cout << ans;
}