def floor_sum(n, m, a, b) :
	# sum_{i=0}^{n-1} floor((a * i + b) // (m)) を返す
	assert a >= 0
	if a == 0 :
		return (b // m) * n
	elif a >= m :
		ret = floor_sum(n, m, a % m, b)
		ret += (a // m) * (n * (n - 1)) // 2
		return ret
	else :
		L = b // m
		R = (a * (n - 1) + b) // m
		ret = L * n
		ret += (R - L) * (n - 1)
		# L + 1 = (a * x + b) / m -> x = ((L + 1) * m - b) / a
		ret -= floor_sum(R - L, a, m, (L + 1) * m - b)
		return ret
def calc(a, b, c, d) :
	if a * d > b * c :
		return calc(c, d, a, b)
	elif a * d == b * c :
		return -1
	else :
		return b * d - (floor_sum(b * d, 2 * d, 2 * c, d) - floor_sum(b * d, 2 * b, 2 * a, b)) - 1
a, b, c, d = map(int, input().split())
print(calc(a, b, c, d))