#include <stdio.h>

int main()
{
	int A, B, C, D, count = 0;

	if (scanf("%d%d%d%d", &A, &B, &C, &D) != 4) {
		return 1;
	}

	for (int i = A; i <= B; i++) {
		for (int j = C; j <= D; j++) {
			if (i == j) {
				continue;
			}
			count++;
		}
	}

	printf("%d", count);

	return 0;
}