#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
struct BinaryTrie {
int cnt;
bool finish;
BinaryTrie* next[2];
BinaryTrie() { finish = 0; cnt = 0; next[0] = next[1] = NULL; }
~BinaryTrie() {
if (next[0]) delete next[0];
if (next[1]) delete next[1];
}
void Insert(const ll x, const int y) {
cnt++;
if (y < 0) { finish = true; return; }
if (!next[(x>>y)&1]) next[(x>>y)&1] = new BinaryTrie;
next[(x>>y)&1]->Insert(x, y - 1);
}
bool Find(const ll x, const int y) {
if (y < 0) return finish;
if (!next[(x>>y)&1]) return false;
return next[(x>>y)&1]->Find(x, y - 1);
}
bool Delete(const ll x, const int y) {
if (y < 0) finish = false;
else {
if (!next[(x>>y)&1]->Delete(x, y - 1)) {
delete next[(x>>y)&1]; next[(x>>y)&1] = nullptr;
}
}
return --cnt;
}
};
ll n, k;
ll a[101010];
string s[101010];
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> k;
BinaryTrie* trie = new BinaryTrie;
for (int i = 1; i <= n; i++) {
cin >> a[i]; trie->Insert(a[i], 30);
}
ll l = -1; ll r = INT_MAX;
while (l + 1 < r) {
ll m = (l + r) >> 1;
ll cnt = 0;
for (int i = 1; i <= n; i++) {
BinaryTrie* ptr = trie;
ll tmp = m;
for (int j = 30; j >= 0; j--) {
int now = (a[i] & (1 << j)) > 0;
if (tmp >= (1 << j)) {
if (ptr->next[now]) cnt += ptr->next[now]->cnt;
now ^= 1;
tmp ^= (1 << j);
}
if (!ptr->next[now]) break;
ptr = ptr->next[now];
}
}
cnt = (cnt - n) / 2;
if (cnt < k) l = m;
else r = m;
}
cout << r - 1 << '\n';
return 0;
}
