A, B, C, D = map(int, input().split())
set1 = set(range(A, B+1))
set2 = set(range(C, D+1))

ctr = 0

for num in set1:
	if num in set2:
		ctr += len(set2)-1
	else:
		ctr += len(set2)
		
print(ctr)