#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = 0;
//---------------------------------//

int main() {
	int N, K;
	cin >> N >> K;
	
	vector<bool> sieve(N, true);
	vector<int> pfcnt(N, 0), divc(N, 1), cnt(N, 0);
	
	FOR(i, 2, N) {
		if (!sieve[i]) continue;
		int ncnt = 0;
		for (int m = N; m % i == 0; m /= i, ++ncnt);
		
		for (int j = 1, t = i * j; t < N; ++j, t += i) {
			if (j % i > 0) cnt[j] = 0;
			
			cnt[t] = cnt[j] + 1;
			pfcnt[t] += min(cnt[t], ncnt);
			divc[t] *= cnt[t] + 1;
			sieve[t] = false;
		}
	}
	
	int ansc = -1, ans = -1;
	FOR(i, 1, N) if (pfcnt[i] >= K && chmax(ansc, divc[i])) ans = i;
	cout << ans << endl;
	return 0;
}