T = int(input())
#4394 993298361
for j in range(T):
    a,b = map(int,input().split())
    if b%a == 0:
        print(0)
        continue
    elif b//a < 2:
        print(-1)
        continue

    for k in range(b//a,1,-1):
        ok,ng = 0,1000000000

        while abs(ok-ng) > 1:
            mid = abs((ok+ng)//2)
            if (b+mid)//(a+mid) < k:
                ng = mid
            else:
                ok = mid

        if (b+ok)%(a+ok) == 0:
            print(ok)
            break