import math

T = int(input())
for tt in range(T):
    A, P = map(int, input().split())

    isPrime = True
    lim = int(math.sqrt(P) + 1)
    for i in range(2, lim):
        if P % i == 0:
            isPrime = False
            break

    if isPrime:
        print(P - (A % P))
    else:
        print(-1)