#include <iostream>
#include <bits/stdc++.h>
#define Kirlos ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
#define ll long long
#define pb(k) push_back(k)
#define fi first
#define se second
#define endl '\n'
#define mp(x,y) make_pair(x,y)
#define MOD 1000000007
#define all(x) x.begin(), x.end()
#define PI acos(-1)
#define sin(x) sin((x)*PI/180)
#define cos(x) cos((x)*PI/180)
#define tan(x) tan((x)*PI/180)
#define Ones(x) __builtin_popcountll(x)

using namespace std;

void solve()
{
    ll n,k;
    cin>>n>>k;
    vector<ll>div;
    for(int i=1; (ll)i*i<=n; i++)
    {
        if(n%i==0)
        {
            div.push_back(i);
            if(n/i!=i)
            {
                div.push_back(n/i);
            }
        }
    }
    sort(all(div));
    if(k>div.size())
    {
        cout<<-1;
    }
    else
    {
        cout<<div[k-1];
    }
}

int main()
{
    Kirlos
    ll t;
    t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
    return 0;
}