#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <cmath>
#include <complex>
#include <map>
#include <set>
#include <vector>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cassert>
#include <iomanip>
using namespace std;

#define Rep(b, e, i) for(int i = b; i <= e; i++)
#define Repr(e, b, i) for(int i = e; i >= b; i--)
#define rep(n, i) Rep(0, n-1, i)
#define repr(n, i) Repr(n-1, 0, i)
#define all(v) (v).begin(), (v).end()
#define pb(v) push_back(v)
#define uniq(v) (v).erase(unique(all(v)),(v).end())
#define bitcnt(x) __builtin_popcount(x)
#define fst first
#define snd second
#define Pqaz(T) priority_queue<T,vector<T>,greater<T>>
#define Pqza(T) priority_queue<T>
#define put(x) cout << x;
#define putsp(x) cout << x << ' ';
#define putln(x) cout << x << endl;
#define ENJYU std::ios::sync_with_stdio(false);std::cin.tie(0);

typedef long long ll;
typedef pair<ll, ll> llP;
typedef pair<int, int> intP;
typedef complex<double> comp;
typedef vector <int> vec;
typedef vector <ll> vecll;
typedef vector <double> vecd;
typedef vector <vec> mat;
typedef vector <vecll> matll;
typedef vector <vecd> matd;

//vector の中身を出力
template <class T>ostream &operator<<(ostream &o,const vector<T>&v)
{o<<"{";for(int i=0;i<(int)v.size();i++)o<<(i>0?", ":"")<<v[i];o<<"}";return o;}

const int MAX = 200000;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = 1<<29;
const ll INFL = 1e18;
const int MOD = 1000000007;

int dx4[4]={1,0,-1,0};
int dy4[4]={0,1,0,-1};
int dx8[8]={1,0,-1,1,-1,1,0,-1};
int dy8[8]={1,1,1,0,0,-1,-1,-1};

//********************************template END****************************************//



void solve(void){

	int N, M;
	cin >> N >> M;

	vecll as(N-1);

	ll A; cin >> A;

	rep(N-1, i) cin >> as[i];

	if (N == 2)
	{
		cout << as[0] << endl;
		return;
	}

	sort(all(as));

	if (as[N-2] + A < as[N-3] + as[N-4] && M == 1)
	{
		cout << -1 << endl;
		return;
	}

	int ng = -1, ok = N-2;

	while (abs(ng-ok) > 1)
	{

		int mid = (ok+ng) / 2;

		ll score = as[mid] + A;

		vecll copy = as;
		copy[mid] = INFL;

		sort(all(copy));

		int cnt = 0;

		repr(N-1, i)
		{
			if (copy[i] + copy[i-1] > score)
			{
				cnt++;
				i--;
			}
			else
			{
				break;
			}
		}

		(cnt <= M-1 ? ok : ng) = mid;

		//cout << cnt << ' ' << mid << endl;

	}

	cout << as[ok] << endl;

}

int main(void){
	solve();
	//cout << "yui(*-v・)yui" << endl;
	return 0;
}