#include <fstream>
#include <iostream>


int main() {

	//std::ifstream inf("Text.txt");	std::cin.rdbuf(inf.rdbuf());

	int A, B, C, D;
	std::cin >> A >> B >> C >> D;
	
	int ans = 0;

	for (int i = A; i <= B; i++) {
		for (int j = C; j <= D; j++) {
			if (i != j) {
				ans++;
			}
		}
	}

	std::cout << ans << std::endl;

	return 0;

}