from heapq import heappush, heappop, heapify
from collections import defaultdict, deque,Counter
from math import ceil, floor, sqrt, factorial,gcd,cos,sin,pi
from itertools import permutations, combinations,product
from bisect import bisect_left, bisect_right
from copy import deepcopy
from fractions import Fraction
from random import randint
import sys
from functools import cache,lru_cache #@lru_cache(maxsize=None)
from time import time
#from sortedcontainers import SortedList # type: ignore
#sys.setrecursionlimit(10**6)
vector1 = [[0, -1], [1, 0], [0, 1], [-1, 0]]
vector2 = [[0, 1], [1, 0], [-1, 0], [0, -1],
           [1,-1], [-1, 1], [1, 1], [-1, -1]]
alphabet = "abcdefghijklmnopqrstuvwxyz"


def make_divisors(n):
    lower_divisors, upper_divisors = [], []
    i = 1
    while i * i <= n:
        if n % i == 0:
            lower_divisors.append(i)
            if i != n // i:
                upper_divisors.append(n//i)
        i += 1
    return lower_divisors + upper_divisors[::-1]


def main():
    
    for _ in range(int(input())):
        a,b = map(int,input().split())
        if b%a == 0:
            print(0)
            continue
        
        divisors = make_divisors(b-a)
        f = True
        for divisor in divisors:
            if divisor > a:
                print(divisor-a)
                f = False
                break
        if f:
            print(-1)
        
        

if __name__ == '__main__':
    main()