#include<iostream>

#include <algorithm>

#include <queue>

#include <string>

using namespace std;

typedef long long  ll;

const int inf = 0x3f3f3f3f;

const int moder = 1e9 + ;

const int MAXN=;

int main()

{

    ll n,k;

    cin >> n >> k;

    vector<ll>v;

    for(ll i=;i <= sqrt(n);i++)

    {

        if(n%i == ) v.push_back(i);

        if(n%i == &&i*i != n) v.push_back(n/i);

    }

    sort(v.begin(),v.end());

    if(k >= v.size()) puts("-1");

    else cout << v[k-] << endl;

    return ;

}