// yukicoder: No.799 赤黒かーどげぇむ
// 2019.5.4 bal4u

#include <stdio.h>

int main()
{
	int A, B, C, D, ans;
	scanf("%d%d%d%d", &A, &B, &C, &D);
	ans = (B-A+1)*(D-C+1);
	if (A < C) A = C;
	if (B > D) B = D;
	if (B >= A) ans -= (B-A+1);
	printf("%d\n", ans);
	return 0;
}