#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
const int inf=0x3f3f3f3f;

#define FOR(i,b,e) for(int i=b; i<=e; i++)
#define FORD(i,b,e) for(int i=b; i>=e; i--)
#define SIZE(x) ((int)x.size())
#define pb push_back
#define st first
#define nd second
#define sp ' '
#define ent '\n'

const int N=2e5+6;
const int M=29;

int n, k;
int t[N];
set<int> ok[M+1], nieok[M+1];
bool usuw[N];

void usun(int i){
	usuw[i]=1;
	FOR(j, 0, M){
		if(t[i] & (1<<j)){
			if(ok[j].find(i)!=ok[j].end()) ok[j].erase(i);
		}
		else{
			if(nieok[j].find(i)!=nieok[j].end()) nieok[j].erase(i);
		}
	}
}

void solve(){
	cin>>n>>k;
	FOR(i, 1, n) cin>>t[i];
	FOR(i, 1, n){
		FOR(j, 0, M){
			if(t[i] & (1<<j)) ok[j].insert(i);
			else nieok[j].insert(i);
		}
	}
	vi xd;
	FORD(j, M, 0){
		if(SIZE(ok[j])>=k){
			for(auto &x: nieok[j]) xd.pb(x);
			for(auto &x: xd) usun(x);
			xd.clear();
		}
	}
	int ans=-1;
	FOR(i, 1, n) if(!usuw[i]) ans&=t[i];
	cout<<ans<<ent;
}

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	solve();
	return 0;
}