#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  ll k;
  cin >> n >> k;
  VI a(n);
  rep(i, n) cin >> a[i];
  sort(all(a));
  k = 2 * k + n;
  int ans = 0;
  struct S {
    int l1, r1, l2, r2;
  };
  vector<S> d{S{0, n, 0, n}}, nd;
  rrep(i, 30) {
    auto get_mid = [&](int l, int r) -> int {
      assert(l < r);
      return ranges::lower_bound(a.begin() + l, a.begin() + r, a[l] >> i | 1, {}, [&](int x) { return x >> i; }) - a.begin();
    };
    ll cnt = 0;
    for (auto [l1, r1, l2, r2] : d) {
      int c1 = get_mid(l1, r1), c2 = get_mid(l2, r2);
      cnt += ll(c1 - l1) * (c2 - l2) + ll(r1 - c1) * (r2 - c2);
    }
    nd.clear();
    if (cnt >= k) {
      for (auto [l1, r1, l2, r2] : d) {
        int c1 = get_mid(l1, r1), c2 = get_mid(l2, r2);
        if (l1 < c1 && l2 < c2) nd.emplace_back(l1, c1, l2, c2);
        if (c1 < r1 && c2 < r2) nd.emplace_back(c1, r1, c2, r2);
      }
    } else{
      ans |= 1 << i;
      k -= cnt;
      for (auto [l1, r1, l2, r2] : d) {
        int c1 = get_mid(l1, r1), c2 = get_mid(l2, r2);
        if (l1 < c1 && c2 < r2) nd.emplace_back(l1, c1, c2, r2);
        if (c1 < r1 && l2 < c2) nd.emplace_back(c1, r1, l2, c2);
      }
    }
    swap(d, nd);
  }
  cout << ans << '\n';
}