#include <stdio.h>

int main(void) {
	int A, B, C, D, ans = 0;
	scanf("%d%d%d%d", &A, &B, &C, &D);
	for (int i = A; i <= B; ++i) {
		for (int j = C; j <= D; ++j) {
			if (i != j) ++ans;
		}
	}
	printf("%d", ans);
}