T = int(input())
NK = [tuple(map(int, input().split())) for _ in range(T)]

# エラトステネスの篩
def eratos(N,K):
    primes = [True] * (N+1)
    primes[0], primes[1] = False, False
    for i in range(1, K+1):
        a = i
        if a * a > N:break
        b = 1
        for _ in range(K):
                primes[a] = False
                a += i
                if a > N:break
    ret = [num for num, is_prime in enumerate(primes) if is_prime]
    return ret


for N,K in NK:
    P = eratos(N,K)
    if len(P) == 0:
         print(-1)
    else:
         print(P[-1])