from bisect import bisect_left

def make_divisors(n):
    divisors = []  # 必要に応じてsetにしても良いかも
    i = 1
    while i ** 2 <= n:
        if n % i == 0:
            divisors.append(i)
            if i ** 2 != n:
                divisors.append(n // i)
        i += 1
    divisors.sort()
    return divisors

def solve(A, B):
    C = B - A
    if B % A == 0:
        return 0
    if B - 2 * A < 0:
        return -1
    X = make_divisors(C)
    y = bisect_left(X, A)
    if y > len(X):
        return -1
    return X[y] - A

for _ in range(int(input())):
    A, B = map(int, input().split())
    print(solve(A, B))