#include <bits/stdc++.h>
#define ll long long
#define int ll
#define vi vector<int>
#define rep(i,s,e) for(int i=(s); i<=(e); i++)
#define arr2 array<int,2>
#define arr3 array<int,3>
using namespace std;
const int LM=100100, asdf=107, fdsa=1e9+7;
int N, k;
int A[LM];
int Naive(){
vi a;
rep(i,1,N){
rep(j,i+1,N) a.push_back(A[i]^A[j]);
}
sort(a.begin(), a.end());
return a[k-1];
}
unordered_map<int,int>m[31];
int Q(int M){
int ans=0;
rep(i,1,N){
for(int j=30; j>=0; j--){
if(M>>j&1){
int q= ((A[i]^M)>>j) ^ 1 ;
ans += m[j][q];
}
if(m[j][ (A[i]^M)>>j ] ==0) break;
}
}
return (ans-N)/2;
}
signed main() {
ios_base::sync_with_stdio(0);cin.tie(0);
cin>>N>>k;
rep(i,1,N) cin>>A[i];
rep(i,1,N){
rep(j,0,30){
m[j][ A[i]>>j]++;
}
}
int low=0, hig=(1ll<<31)-1;
while(low<hig){
int mid=low+hig>>1;
if(Q(mid) < k) low = mid+1;
else hig=mid;
}
cout<<low-1;
}
