T = int(input())

for _ in range(T):
	a,b = map(int,input().split())
	c = b - a
	if c == 0:
		print(0)
		continue
	if c < a:
		print(-1)
	else:
		s = set()
		for i in range(1,10 ** 5):
			if c % i == 0:
				s.add(i)
				s.add(c // i)
		s.add(c)
		ans = 10 ** 20
		for k in s:
			if k >= a and k - a < ans:ans = k - a
		print(ans)