#include <bits/stdc++.h>

using namespace std;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n, m;
	cin >> n >> m;
	int x;
	cin >> x;
	vector<int> a(n - 1);
	for (int i = 0; i < n - 1; i++) cin >> a[i];
	sort(a.begin(), a.end());
	int ng = -1, ok = n - 1;
	while (ok - ng > 1) {
		int mid = (ok + ng) / 2;
		int tar = x + a[mid];
		int cnt = 0;
		for (int i = 0, j = n - 2; i < n - 1; i++) {
			if (i == mid) continue;
			if (j == mid) j--;
			if (i >= j) break;
			if (a[i] + a[j] > tar) {
				cnt++;
				j--;
			}
		}
		if (cnt >= m) ng = mid;
		else ok = mid;
	}
	cout << (ok != n - 1 ? a[ok] : -1) << endl;
	return 0;
}