#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

struct Eratosthenes {
    vector<bool> isprime;
    vector<int> primes;
    vector<int> spf; // smallest prime factors
    vector<int> mobius;

    Eratosthenes(int N) : isprime(N + 1, true),
                          spf(N + 1, -1),
                          mobius(N + 1, 1) {
        isprime[1] = false;
        spf[1] = 1;

        for(int p = 2; p <= N; p++){
            if(!isprime[p]) continue;
            primes.push_back(p);
            spf[p] = p;
            mobius[p] = -1;
            for(int q = p * 2; q <= N; q += p){
                isprime[q] = false;
                if(spf[q] == -1) spf[q] = p;
                mobius[q] = ((q / p) % p == 0 ? 0 : -mobius[q]);
            }
        }
    }

    vector<pair<int,int>> factorize(int n) {
        vector<pair<int,int>> res;
        while(n > 1) {
            int p = spf[n], e = 0;
            while(spf[n] == p) n /= p, e++;
            res.push_back({p, e}); // p^e
        }
        return res;
    }

    vector<int> divisors(int n) {
        vector<int> res({1});
        auto pf = factorize(n);
        for(auto p : pf) {
            int s = (int)res.size();
            for(int i = 0; i < s; i++) {
                int v = 1;
                for(int j = 0; j < p.second; j++) {
                    v *= p.first;
                    res.push_back(res[i] * v);
                }
            }
        }
        return res;
    }

    template<class T> void fast_zeta(vector< T > &f) {
        int N = f.size();
        vector<bool> isprime = Eratosthenes(N);
        for(int p = 2; p < N; p++) {
            if(!isprime[p]) continue;
            for(int k = (N - 1) / p; k >= 1; k--) {
                f[k] += f[k * p];
            }
        }
    }

    template<class T> void fast_mobius(vector< T > &F) {
        int N = F.size();
        vector<bool> isprime = Eratosthenes(N);
        for(int p = 2; p < N; p++) {
            if(!isprime[p]) continue;
            for(int k = 1; k * p < N; k++) {
                F[k] -= F[k * p];
            }
        }
    }

    template<class T> vector< T > gcd_convolution(const vector< T > &f, const vector< T > &g) {
        int N = max(f.size(), g.size());
        vector< T > F(N, 0), G(N, 0), H(N);
        for(int i = 0; i < f.size(); i++) F[i] = f[i];
        for(int i = 0; i < g.size(); i++) G[i] = g[i];

        fast_zeta(F);
        fast_zeta(G);

        for(int i = 1; i < N; i++) H[i] = F[i] * G[i];

        fast_mobius(H);
        return H;
    }

    long long fast_euler_phi(int n) {
        auto pf = factorize(n);
        long long res = n;
        for(auto p : pf) {
            res *= p.first - 1;
            res /= p.first;
        }
        return res;
    }
};

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N,K; cin >> N >> K;
    Eratosthenes sieve(N);
    auto pf_N = sieve.factorize(N);
    vector<int> target(N + 1, 0);
    int M = -1, dM = 0;
    for(auto [p, e] : pf_N) target[p] = e;
    for(int i = 1; i < N; i++) {
        auto pf_i = sieve.factorize(i);
        int cnt = 0, di = 1;
        for(auto [p, e] : pf_i) {
            cnt += min(target[p], e);
            di *= e + 1;
        }
        if(cnt >= K && dM < di) {
            M = i;
            dM = di;
        }
    }
    cout << M << endl;
}