import bisect

def divisors(n):
    lower, upper = [], []
    i = 1
    while i*i <= n:
        if n % i == 0:
            lower.append(i)
            if i != n // i:
                upper.append(n//i)
        i += 1

    return lower + upper[::-1]

t = int(input())

for i in range(t):
    a, b = map(int, input().split())
    if a == b:
        print(0)
    else:
        ba = b - a
        ans = divisors(ba)
        j = bisect.bisect_left(ans, a)
        if len(ans[j:]) == 0:
            print(-1)
        else:
            print(min(ans[j:]) - a)