from bisect import bisect_left

T = int(input())

def divisor(n):
    ans = []
    for i in range(1, int(n**0.5)+1):
        if n % i == 0:
            ans.append(i)
            if i*i != n:
                ans.append(n//i)
    return sorted(ans)

for _ in range(T):
    A, B = map(int, input().split())
    if B % A == 0:
        print(0)
        continue
    diff = B-A
    if diff < A:
        print(-1)
    else:
        D = divisor(diff)
        b = bisect_left(D, A)
        if b < len(D):
            print(D[b]-A)
        else:
            print(-1)