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

is_prime = [False for _ in range(5000001)]
for i in range(2, 5000001):
	if not is_prime[i]:
		is_prime[i] = True
		for j in range(2*i, 5000001, i):
			is_prime[j] = True
	else:
		is_prime[i] = False

for _ in range(int(input())):
	a, p = list(map(int, input().split()))
	if is_prime[p]:
		print((-a)%p)
	else:
		print(-1)