#include<iostream>
#include<vector>
#include<algorithm>
#include<cassert>
using namespace std;
int A[2<<17];
int ans=0;
int mid(int l,int r,int k)
{
	int m=l;
	while(m<r&&!(A[m]>>k&1))m++;
	return m;
}
void solve2(vector<pair<int,int> >A,vector<pair<int,int> >B,long K,int k)
{
	if(k==-1)return;
	vector<pair<int,int> >zero,one;
	long v=0;
	for(int i=0;i<A.size();i++)
	{
		auto[al,ar]=A[i];
		auto[bl,br]=B[i];
		if(al==ar||bl==br)continue;
		int am=mid(al,ar,k),bm=mid(bl,br,k);
		zero.push_back(make_pair(al,am));
		one.push_back(make_pair(bl,bm));
		zero.push_back(make_pair(am,ar));
		one.push_back(make_pair(bm,br));
		v+=(long)(am-al)*(bm-bl);
		v+=(long)(ar-am)*(br-bm);
	}
	if(v>=K)solve2(zero,one,K,k-1);
	else
	{
		ans|=1<<k;
		for(int i=0;i<zero.size();i+=2)swap(one[i],one[i+1]);
		solve2(zero,one,K-v,k-1);
	}
}
void solve(vector<pair<int,int> >LR,long K,int k)
{
	if(k==-1)return;
	vector<pair<int,int> >zero,one;
	long v=0;
	for(auto[l,r]:LR)if(l<r)
	{
		int m=mid(l,r,k);
		zero.push_back(make_pair(l,m));
		one.push_back(make_pair(m,r));
		v+=(long)(m-l)*(m-l-1)/2;
		v+=(long)(r-m)*(r-m-1)/2;
	}
	if(v<K)
	{
		ans|=1<<k;
		solve2(zero,one,K-v,k-1);
	}
	else
	{
		for(auto v:one)zero.push_back(v);
		solve(zero,K,k-1);
	}
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int N;
	long K;
	cin>>N>>K;
	for(int i=0;i<N;i++)cin>>A[i];
	sort(A,A+N);
	vector<pair<int,int> >fst;
	fst.push_back(make_pair(0,N));
	solve(fst,K,29);
	cout<<ans<<endl;
}