#include <bits/stdc++.h>
#define int long long
//#define USE_FREOPEN
//#define MUL_TEST
#define FILENAME "prize"
using namespace std;
int a[100005];

void solve() {
	int n,m;
	cin >> n >> m;
	for (int i = 1; i <= n; i++)
		cin >> a[i];
	sort(a + 2,a + n + 1);
	
	auto chk = [&](int x) -> bool {
		int l = 2,r = n,k = 0;
		while (l < r) {
			if (l == x) l++;
			else if (r == x) r--;
			else if (a[l] + a[r] > a[1] + a[x]) k++,l++,r--;
			else l++;
		}
		return k < m;
	};
	
	int l = 1,r = n + 1,ans = -1;
	while (r - l > 1) {
		int mid = l + r >> 1;
		if (chk(mid)) r = mid,ans = mid;
		else l = mid;
	}
	cout << (ans == -1 ? -1 : a[ans]) << '\n';
}

signed main() {
	#ifdef USE_FREOPEN
		freopen(FILENAME ".in","r",stdin);
		freopen(FILENAME ".out","w",stdout);
	#endif
	int _ = 1;
	#ifdef MUL_TEST
		cin >> _;
	#endif
	while (_--)
		solve();
	_^=_;
	return 0^_^0;
}