#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(int i=0; i<int(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
using namespace std;
#include <atcoder/modint>
using Modint = atcoder::static_modint<998244353>;

void testcase(){
    i64 N; cin >> N;
    i64 K; cin >> K;
    K *= 2; K += N;
    vector<i64> A(N); rep(i,N) cin >> A[i];
    sort(A.begin(), A.end());
    vector<i64> L(N), R(N,N);
    i64 ans = 0;
    for(i64 b=29; b>=0; b--){
        vector<i64> M(N);
        rep(i,N) M[i] = lower_bound(A.begin(), A.end(),
            ((A[i] ^ ans) & ~((1ll << b) - 1)) | (1ll << b)) - A.begin();
        i64 k = 0;
        rep(i,N){
            if(A[i] & (1ll << b)) k += R[i] - M[i];
            else k += M[i] - L[i];
        }
        //cout << "ans = " << ans << " , k = " << k << endl;
        if(k < K){
            rep(i,N) ((A[i] & (1ll << b)) ? R[i] : L[i]) = M[i];
            ans |= 1ll << b;
            K -= k;
        } else {
            rep(i,N) ((A[i] & (1ll << b)) ? L[i] : R[i]) = M[i];
        }
    }
    cout << ans << '\n';
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    testcase();
    return 0;
}