import math
import sys


def input():
   return sys.stdin.buffer.readline()[:-1]

MAXX = 5*10**6 + 5

isprime = [1]*MAXX
#theratos
for num in range(2, MAXX):
    if isprime[num]:
        for k in range(num*2, MAXX, num):
            isprime[k] = 0

T = int(input())
for tt in range(T):
    A, P = map(int, input().split())
    if isprime[P]:
        print(P - (A % P))
    else:
        print(-1)