#include<bits/stdc++.h>

using namespace std;

#define MAX 3000000

typedef long long ll;

ll i,l,n,k;

ll a[MAX+];

template <typename T> inline void read(T &x) {

        int f = ; x = ;

        char c = getchar();

        for (; !isdigit(c); c = getchar()) { if (c == '-') f = -f; }

        for (; isdigit(c); c = getchar()) x = x *  + c - '';

        x *= f;

}

template <typename T> inline void write(T x) {

    if (x < ) { putchar('-'); x = -x; }

    if (x > ) write(x/);

    putchar(x%+'');

}

template <typename T> inline void writeln(T x) {

    write(x);

    puts("");

}

int main() {

        read(n); read(k);

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

                if (!(n % i)) {

                        a[++l] = i;

                        if (i * i != n) a[++l] = n / i;

                }

        }

        sort(a+,a+l+);

        if (k <= l) writeln(a[k]);

        else puts("-1");

        return ;

}