#include<stdio.h>
int main()
{
	int a, b, c, d;
	scanf("%d %d %d %d", &a, &b, &c, &d);
	int ans = (b - a + 1) * (d - c + 1);
	if (c > a)a = c;
	if (d < b)b = d;
	if (a <= b)
		ans -= b - a + 1;
	printf("%d\n", ans);
	return 0;
}