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

using ll = long long;
bool deb = 1;
struct Binary_Trie {
  ll base = 36; // 桁数
  struct Node {
    ll ne[2];
    ll c, num;
    Node(ll c_) : c(c_), num(0) { ne[0] = ne[1] = -1; }
  };

  vector<Node> nd;
  ll rt = 0;

  Binary_Trie() { nd.push_back(Node(rt)); }

  void insert(const ll &num) {
    ll id = 0;
    for(ll b = base; b >= 0; b--) {
      ll nt = 0;
      nt = ((num>>b)&1);
      if(nd[id].ne[nt] == -1) {
        nd.push_back(Node(nt));
        nd[id].ne[nt] = nd.size() - 1;
      }
      nd[id].num++;
      id = nd[id].ne[nt];
    }
    nd[id].num++;
  }

  bool search(const ll &num) {
    ll id = 0;
    bool EX = 1;
    for(ll b = base; b >= 0; b--) {
      ll nt = 0;
      nt = ((num>>b)&1);
      if(nd[id].ne[nt] == -1
         || nd[nd[id].ne[nt]].num == 0) {
        EX = 0;
        break;
      }
      id = nd[id].ne[nt];
    }
    return EX;
  }

  void erase(const ll &num) {
    if(!search(num)) return;
    ll id = 0;
    for(ll b = base; b >= 0; b--) {
      ll nt = 0;
      nt = ((num>>b)&1);
      nd[id].num--;
      id = nd[id].ne[nt];
    }
    nd[id].num--;
  }

  ll small_num(const ll &p, const ll &x) { // d XOR p < x なる d の個数
    ll res = 0;
    ll id = 0;
    for(ll b = base; b >= 0; b--) {
      ll nx = 0, np = 0;
      nx = ((x>>b)&1);
      np = ((p>>b)&1);
      if(nx == 1) {
        if(nd[id].ne[np] != -1) {
          res += nd[nd[id].ne[np]].num;
        }
        if(nd[id].ne[1 - np] == -1) { break; }
        else {
          id = nd[id].ne[1 - np];
          continue;
        }
      }
      else {
        if(nd[id].ne[np] == -1) { break; }
        else {
          id = nd[id].ne[np];
          continue;
        }
      }
    }
    return res;
  }

  ll min_XOR(ll x) {
    ll res = 0, id = 0;
    for(ll b = base; b >= 0; b--) {
      ll nx = 0;
      nx = ((x>>b)&1);
      if(nd[id].ne[nx] != -1
         && nd[nd[id].ne[nx]].num > 0) {
        id = nd[id].ne[nx];
      }
      else {
        res += (1ll << b);
        id = nd[id].ne[1 - nx];
      }
    }
    return res;
  }

  ll K_thsmall(ll K) { // K番目に小さい値を返す.(0index)
    if(nd[0].num <= K) return -1e18;
    ll id = 0, res = 0;
    for(ll b = base; b >= 0; b--) {
      if(nd[id].ne[0] != -1
         && nd[nd[id].ne[0]].num > K) {
        id = nd[id].ne[0];
      }
      else {
        res += (1ll << b);
        if(nd[id].ne[0] != -1) {
          K -= nd[nd[id].ne[0]].num;
        }
        id = nd[id].ne[1];
      }
    }
    return res;
  }
};


int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    ll N=2e5;
    ll K=(N*(N-1))/4;
    cin>>N>>K;
    vector<ll> A(N);
    Binary_Trie B;
    for(int i=0;i<N;i++){
        cin>>A[i];
        // A[i]=rand()%ll(1e9);
        B.insert(A[i]);
    }
    ll L=-1,R=4e9;
    while(R-L>1){
        ll M=(R+L)/2;
        ll cnt=0;
        if(M!=0)for(int i=0;i<N;i++){
            cnt+=B.small_num(A[i],M)-1;
        }
        (cnt/2<K?L:R)=M;
    }
    cout<<L<<endl;
}