#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if (y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if (x < y) x = y; }

int main() {
	int N; int M;
	while (~scanf("%d%d", &N, &M)) {
		int a0;
		scanf("%d", &a0);
		vector<int> a(N - 1);
		for (int i = 0; i < N - 1; ++ i)
			scanf("%d", &a[i]);
		sort(a.begin(), a.end());
		int lo = 0, up = N - 1;
		while (up - lo > 0) {
			int mid = (lo + up) / 2;
			int x = a0 + a[mid];
			vector<int> b = a;
			b.erase(b.begin() + mid);
			int num = 0;
			for (int i = (int)b.size() - 1, j = 0; i > j; -- i) {
				for (; j<i&&b[j] + b[i] <= x; ++ j);
				if (j < i) {
					++ j;
					++ num;
				}
			}
			//cerr<<mid<<", "<<x<<": "<<
			if (num >= M)
				lo = mid + 1;
			else
				up = mid;
		}
		printf("%d\n", up == N - 1 ? -1 : a[up]);
	}
	return 0;
}