#include<cstdio>
#include<vector>
#include<algorithm>
#include<functional>
//#define scanf scanf_s
#define fir first
#define sec second
#define mp make_pair
#define mt make_tuple
#define pub push_back
using namespace std;
typedef long long int llint;
const llint one = 1;
const llint big = (one<<30);
const int mod=1000000007;
inline int gcd(int a,int b) { return b ? gcd(b, a % b) : a; }
int main(void){
	int N,M,me,q,gen=0,bmax,bmin,can;
	vector<int> po;
	scanf("%d %d",&N,&M);
	bmax=N-2;
	bmin=-1;
	scanf("%d",&me);
	for(int i=1;i<N;i++){
		scanf("%d",&q);
		po.pub(q);
	}
	sort(po.rbegin(),po.rend());
	//ペアの相手の順位を最大化
	while(bmax-bmin>1){
		gen=(bmax+bmin)/2;
		int dow=N-2,lead=0,tei=me+po[gen],ok=1;
		for(lead=0;lead<M;lead++){
			for(;dow>=0;dow--){
				if(dow==gen){continue;}
				if(po[dow]+po[lead]>tei){break;}
				if(dow<M){ok=0;break;}
			}
			if(ok==0){break;}
		}
		if(ok==1){bmax=gen;}
		else{bmin=gen;}
	}
	gen--;
	if(gen==-1){printf("-1\n");}
	else{printf("%d\n",po[gen]);}
 	return 0;
}