#include<bits/stdc++.h>
using namespace std;
int a[100005],n,m;
bool c(int x){
	int cnt=0;
	int X=a[1]+a[x];
	for(int i=n;i>=2;i-=2){
		if(i==x){
			i++;continue;
		}else if(i-1==x){
			if(a[i]+a[i-2]>X) cnt++;
			i--;
			continue;
		}if(a[i]+a[i-1]>X) cnt++;
	}return cnt<m;
}
int main(){
	cin>>n>>m;
	for(int i=1;i<=n;i++) scanf("%d",&a[i]);
	sort(a+2,a+n+1);
	int l=2,r=n,mid;
	while(l<=r){
		mid=(l+r)/2;
		if(c(mid)){
			r=mid-1;
		}else l=mid+1;
	}if(l>n)cout<<-1;
	else cout<<a[l];
}