#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;

	int res = 0;
	for (int i = a; i <= b; ++i) {
		for (int j = c; j <= d; ++j) {
			if (i != j) { ++res; }
		}
	}

	cout << res << endl;

	return 0;
}