#include<map>
#include<set>
#include<list>
#include<cmath>
#include<queue>
#include<stack>
#include<cstdio>
#include<string>
#include<vector>
#include<complex>
#include<cstdlib>
#include<cstring>
#include<numeric>
#include<sstream>
#include<iostream>
#include<algorithm>
#include<functional>
 
#define mp       make_pair
#define pb       push_back
#define all(x)   (x).begin(),(x).end()
#define YES() printf("YES\n")
#define NO() printf("NO\n")
 
using namespace std;
 
#define int long long
//typedef    long long          ll;
typedef    unsigned long long ull;
typedef    vector<bool>       vb;
typedef    vector<int>        vi;
typedef    vector<vb>         vvb;
typedef    vector<vi>         vvi;
typedef    pair<int,int>      P;
 
const int INF=1e+9;
const double EPS=1e-9;
const int MOD=1000000007;
 
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};

int n,m,a[100000];

bool C(int x){
	int pt = a[0] + a[x],low = 1,up = n - 1,cnt = 0;
	while(up > low){
		if(up == x) up--;
		if(up < 0) break;
		while(1) {
			if(low != x && a[up] + a[low] > pt) break;
			if(up <= low) break;
			low++;
		}
		if(up <= low) break;
		cnt++;
		up--;
		low++;
	}
	return cnt < m;
}

signed main(){
	cin >> n >> m;
	for(int i = 0;i < n;i++) cin >> a[i];
	if(n == 2){
		cout << a[1] << endl;
		return 0;
	}
	sort(a + 1,a + n);
	int low = 1,up = n;
	while(up - low > 1){
		int mid = (low + up) / 2;
		if(C(mid)) up = mid;
		else low = mid;
	}
	if(up >= n) cout << -1 << endl;
	else cout << a[up] << endl;
	return 0;
}