def divisors(n, a):
    result = []
    i = 1
    while i*i <= n:
        if n % i == 0:
            if i >= a:
                result.append(i)
            if i*i < n and n >= a*i:
                result.append(n//i)
        i += 1
    return sorted(result)


def solve(a, b):
    if a == b:
        return 0
    d = divisors(b-a, a)
    if d:
        return d[0]-a
    else:
        return -1


for t in range(int(input())):
    print(solve(*[int(a) for a in input().split()]))