#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long ll;
int a, b, c, d;

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