def run():
	A, B, C, D = [int(x) for x in input().split()]
	X = B - A + 1
	Y = D - C + 1
	
	if B < C or D < A:
		Z = 0
	else:
		Z = B - C if D > A else D - A

	print(X * Y - Z)

if __name__ == '__main__':
	run()