from bisect import bisect_left
T = int(input())
for _ in range(T):
    a,b = map(int,input().split())
    n = b-a
    if n==0:
        print(0)
    else:
        A = set()
        for i in range(1,n+1):
            if i*i>n:break
            if n%i==0:
                A.add(i)
                A.add(n//i)
        A = sorted(list(A))
        ind = bisect_left(A,a)
        if ind==len(A):
            print(-1)
        else:
            print(A[ind]-a)