INF = 10**18
from bisect import bisect_left
T = int(input())

def num_ijou(li, x): # x 以上の最小値（なければ inf）
    res = bisect_left(li, x)
    return INF if res == len(li) else li[res]

def make_divisors(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):
    if a == b:
        return 0
    # (B - A)/(A + X) が割り切れれば良い
    divs = make_divisors(b - a)
    # divs の中から a 以上の最小値を見つける

    memo = num_ijou(divs, a)
    if memo == INF:
        return -1
    else:
        return memo - a

ansl = []
for i in range(T):
    a, b = map(int, input().split())
    ansl.append(solve(a, b))

print(*ansl,sep='\n')