#include <iostream>
using namespace std;
int main()
{
	int A, B, C, D,c=0;
	cin >> A >> B >> C >> D;
	for (int i = A; i <= B; i++)
	{
		for (int j = C; j <= D; j++)
		{
			if (i != j) c++;
		}
	}
	cout << c << endl;
	return 0;
}