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

void fast_io() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
}

int main() {
	fast_io();
	int n;
	long long k;
	cin >> n >> k;
	vector<int> a(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	k--;
	int ans = 0;
	for (int i = 30; i >= 0; i--) {
		map<int, long long> cnt;
		int msk = ~((1 << i) - 1);
		for (int j = 0; j < n; j++) {
			cnt[a[j] & msk]++;
		}
		long long cur = 0;
		for (auto [key, val] : cnt) {
			if ((key ^ ans) > key && cnt.count(key ^ ans)) {
				cur += val * cnt[key ^ ans];
			} else if (ans == 0) {
				cur += val * (val - 1) / 2;
			}
		}
		if (cur <= k) {
			ans |= 1 << i;
			k -= cur;
		}
	}
	cout << ans << endl;
}