#include "bits/stdc++.h"

using namespace std;

void solve()
{
	int a, b, c, d, ans = 0;
	cin >> a >> b >> c >> d;
	for (int i = a; i <= b; i++)
	{
		for (int j = c; j <= d; j++)
		{
			if (i != j)
			{
				ans++;
			}
		}
	}
	cout << ans << endl;
}

int main(void)
{
	solve();
	//cout << "yui(*-v・)yui" << endl;
	return 0;
}