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

typedef unsigned long long ll;
typedef std::pair<int, int> P;

#define frep(i, a, b) for(int i = (a); i < (b); i++)
#define rep(i, n) frep(i, 0, (n))
#define rrep(i, n) for(int i = (n - 1); i >= 0; i--)
#define all(i, x) for(typeof(x.begin()) i = x.begin(); i != x.end(); i++)
#define pb push_back

const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
const int MAX = 100010;
const int INF = 1e9;
const int MOD = 1000000007;
const string END = "\n";

void solve(){
	int n, m, a[MAX], ans;
	cin >> n >> m;
	rep(i, n) cin >> a[i];
	sort(a, a + n);
	int left = 0, right = n;
	while(right - left > 1){
		int mid = (right + left) / 2;
		int tmp = a[mid] + a[0];
		int ll = 1, rr = n-1, cnt = 0;
		while(ll < rr){
			if(ll == mid) ll += 1;
			else if(rr == mid) rr -= 1;
			else if(a[rr] + a[ll] >= tmp) ll += 1, rr -= 1, cnt += 1;
			else ll += 1;
		}
		if(cnt < m) right = mid;
		else left = mid;
	}
	if(right == n){
		ans = -1;
	}else{
		ans = a[right];
	}
	cout << ans << END;
}

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