from math import gcd

n, k = map(int, input().split())

primes = [1] * (n + 1)
cnt_s = [0] * (n + 1)
primes[0] = 0
for i in range(2, n + 1):
    if primes[i] != 1:
        continue
    for j in range(i, n + 1, i):
        cnt = 0
        J = j
        while j % i == 0:
            j //= i
            cnt += 1
        primes[J] *= (cnt + 1)
        cnt_s[J] += cnt
        
max_ = 0
ans = -1
for i in range(1, n):
    g = gcd(n, i)
    if cnt_s[g] < k:
        continue
    if primes[i] > max_:
        max_ = primes[i]
        ans = i
print(ans)