def makediv(n):
	lower_divisors , upper_divisors = [], []
	i = 1
	while i*i <= n:
		if n % i == 0:
			lower_divisors.append(i)
			if i != n // i:
				upper_divisors.append(n//i)
		i += 1
	return lower_divisors + upper_divisors[::-1]

def solve():
	a, b = map(int,input().split())
	if b % a == 0:
		print(0)
		return
	d = makediv(b - a)
	x = 10 ** 18
	for i in d:
		if i - a < 0: continue
		x = min(x, i - a)
	if x == 10 ** 18:
		print(-1)
	else:
		print(x)


T = int(input())
for _ in range(T):
	solve()