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

def f(dv, a):
    for x in dv:
        if a <= x:
            return x
    return -1


T = int(input())
for t in range(T):
    a, b = map(int, input().split())
    dv = divisors(b-a)
    res = f(dv, a)
    if res == -1:
        print(-1)
    else:
        print(res - a)