def get_divisors(n: int) -> list:
    """
    n の約数を昇順にリストで返す関数
    """
    divisors = set()
    i = 1
    while i * i <= n:
        if n % i == 0:
            divisors.add(i)
            divisors.add(n // i)
        i += 1
    return sorted(divisors)
t=int(input())
for _ in range(t):
    a,b=map(int,input().split())
    if a==b:
        print(0)
    else:
        D=get_divisors(b-a)
        for i in D:
            if i>=a:
                if (b+i-a)%i==0:
                    print(i-a)
                    break
        else:
            print(-1)