#include<iostream>
#include<vector>
#include<string>
#include<algorithm>	
#include<map>
#include<set>
#include<utility>
#include<cmath>
#include<cstring>
#include<queue>
#include<stack>
#include<cstdio>
#include<sstream>
#include<iomanip>
#include<assert.h>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mt make_tuple
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)
using namespace std;
//kaewasuretyuui
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vp;
typedef vector<vp> vvp;
typedef vector<string> vs;
typedef vector<double> vd;
typedef tuple<int,int,int> tp;
typedef vector<tp> vt;
typedef vector<vd> vvd;
typedef pair<int,pii> pip;
typedef vector<pip>vip;
const double PI=acos(-1);
const double EPS=1e-7;
const int inf=1e8;
const ll INF=1e16;
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
int main(){
	int n,m;
	cin>>n>>m;
	int a;cin>>a;
	n--;
	vi in(n);
	rep(i,n)cin>>in[i];
	sort(all(in),[](int a,int b){
		return a>b;
	});
	int l=-1,r=n;
	while(r-l>1){
		int h=(l+r)/2;
		if(h<0){
			r=-1;
			break;
		}
		int sum=a+in[h];
		int L=0,R=2*(m-1)-1;
		if(h<=R)R++;
		bool H=true;
		rep(i,m-1){
			if(h==L)L++;
			if(h==R)R--;
			if(sum>in[R]+in[L])H=false;
			R--;L++;
		}
		if(H)r=h;
		else l=h;
	}
	if(r==-1)cout<<-1<<endl;
	else cout<<in[r]<<endl;
}