#include <bits/stdc++.h>
#include <atcoder/all>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define SELECTER(_1,_2,_3,SELECT,...) SELECT
#define rep1(i,n) for(int i=0;i<int(n);++i)
#define rep2(i,a,n) for(int i=int(a);i<int(n);++i)
#define rep(...) SELECTER(__VA_ARGS__,rep2,rep1)(__VA_ARGS__)
#define RSELECTER(_1, _2, _3, RSELECT, ...) RSELECT
#define rrep1(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define rrep2(i,a,n) for(int i=(int)(n)-1;i>=(int)a;--i)
#define rrep(...) RSELECTER(__VA_ARGS__, rrep2, rrep1)(__VA_ARGS__)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define fi first
#define se second
#define PrintR LogOutput
#ifdef _DEBUG
#define Log(...) LogOutput(__VA_ARGS__)
#else
#define Log(...)
#endif
#define M_PI 3.14159265358979323846
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
using pii=pair<int,int>;
using pll=pair<long long,long long>;
using pdd=pair<long double,long double>;
using tp=tuple<int,int,int>;
using tpll=tuple<ll,ll,ll>;
using veci=vector<int>;
using vecpii=vector<pair<int,int>>;
using vecll=vector<long long>;
using vecpll=vector<pair<long long,long long>>;
using vecpdd=vector<pair<long double,long double>>;
using vecs=vector<string>;
using vecb=vector<bool>;
using vecd=vector<long double>;
using vectp=vector<tp>;
using vectpll=vector<tpll>;
using mint=modint998244353;
using mint10=modint1000000007;
template<typename T, typename S>
istream& operator>>(istream& in, pair<T, S>& a){return in >> a.first >> a.second;}
template<typename T, typename S>
ostream& operator<<(ostream& out, const pair<T, S>& a){return out << a.first << ' ' << a.second;}
ostream& operator<<(ostream& out, const mint& a){return out << a.val();}
ostream& operator<<(ostream& out, const mint10& a){return out << a.val();}
ostream& operator<<(ostream& out, const modint& a){return out << a.val();}
template<typename T>
ostream& operator<<(ostream& out, const vector<T>& d){for(int i = 0 ; i < d.size() ; ++i) out << d[i] << (i == d.size() - 1 ? "" : " "); return out;}
template<typename T, typename S>
pair<T, S> operator+(const pair<T, S>& a, const pair<T, S>& b){return {a.fi + b.fi, a.se + b.se};}
template<typename T, typename S>
pair<T, S> operator-(const pair<T, S>& a, const pair<T, S>& b){return {a.fi - b.fi, a.se - b.se};}
template<class T> inline bool chmax(T& a,T b){if(a<b) {a=b;return true;} return false;}
template<class T> inline bool chmin(T& a,T b){if(a>b) {a=b;return true;} return false;}
bool Judge(int i, int j, int h, int w){return i < 0 || j < 0 || i >= h || j >= w;}
bool PrintA(int i){cout<<(i ? "Yes" : "No")<<endl;return i;}

constexpr ll INF=numeric_limits<ll>::max() >> 2;
constexpr int inf=numeric_limits<int>::max() >> 1;
constexpr ll MOD=998244353;
const int vi[] = {0, 1, 0, -1}, vj[] = {1, 0, -1, 0};

template<typename... Args>
void LogOutput(Args&&... args){
	stringstream ss;
	((ss << args << ' '), ...);
	cout << ss.str().substr(0, ss.str().length() - 1) << endl;
}
template<typename T>
void LogOutput(vector<vector<T>>& data){for(auto d : data) LogOutput(d);}

int main(){
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n;
	ll k;cin>>n>>k;
	veci a(n);
	rep(i, n) cin>>a[i];
	{
		map<ll, int> mp;
		rep(i, n) mp[a[i]]++;
		ll cnt = 0;
		for(auto v : mp) cnt += ll(v.se) * (v.se - 1) / 2;
		if(cnt >= k){
			cout<<0<<endl;
			return 0;
		}
	}
	auto rec = [&](auto f, veci l, veci r, int i, int bs, int u = 0) -> ll{
		if(i == -1){
			if(!u) return 1LL * l.size() * r.size();
			return (ll(l.size()) - 1) * l.size() / 2;
		}
		if(l.empty()) return 0;
		if(!u && r.empty()) return 0;
		veci a, b;
		rep(j, l.size()){
			if(l[j] >> i & 1) b.push_back(l[j]);
			else a.push_back(l[j]);
		}
		ll res = 0;
		if(!u){
			veci c, d;
			rep(j, r.size()){
				if(r[j] >> i & 1) d.push_back(r[j]);
				else c.push_back(r[j]);
			}
			if(bs >> i & 1){
				res = 1LL * a.size() * c.size() + 1LL * b.size() * d.size();
				res += f(f, a, d, i - 1, bs) + f(f, b, c, i - 1, bs);
			}else{
				res = f(f, a, c, i - 1, bs) + f(f, b, d, i - 1, bs);
			}
		}else{
			if(bs >> i & 1){
				res = (ll(a.size()) - 1) * a.size() / 2 + (ll(b.size() - 1)) * b.size() / 2;
				res += f(f, a, b, i - 1, bs);
			}else{
				res += f(f, a, {}, i - 1, bs, u) + f(f, b, {}, i - 1, bs, u);
			}
		}
		return res;
	};
	int l = 0, r = inf;
	while(r - l > 1){
		int mid = (l + r) / 2;
		if(rec(rec, a, {}, 30, mid, 1) >= k) r = mid;
		else l = mid;
	}
	cout<<r<<endl;
}