#include <iostream>

using namespace std;

int a, b, c, d;

int main(void)
{
	cin >> a >> b >> c >> d;
	if(b < c || d < a){
		cout<<(b-a+1)*(d-c+1) << endl;
	}
	else{
		cout << (b-a+1)*(d-c+1) - (min(b, d)-max(a, c)+1) << endl;
	}
	return 0;
}