#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,m;
bool check(int k,int *a){
  vector<int> v;
  v.push_back(a[0]+a[k]);
  int tmp=0;
  for(int i=1;i<n;i++){
    if(i==k) i++;
    if(tmp==0) tmp=a[i];
    else{
      v.push_back(tmp+a[i]);
      tmp=0;
    }
  }
  //cout<<v.size()<<endl;
  //for(int k:v) cout<<"-"<<k<<endl;
  assert((int)v.size()==n/2);
  sort(v.begin(),v.end());
  auto p=upper_bound(v.begin(),v.end(),a[0]+a[k]);
  if(p==v.end()) return 1;
  return ((n/2)-(p-v.begin()))<m;
}
signed main(){
  cin>>n>>m;
  int a[n];
  for(int i=0;i<n;i++) cin>>a[i];
  sort(a+1,a+n);
  int l=1,r=n-1;
  if(!check(r,a)){
    cout<<-1<<endl;
    return 0;
  }
  if(check(l,a)){
    cout<<a[l]<<endl;
    return 0;
  }
  while(l+1<r){
    int mid=(l+r)/2;
    if(check(mid,a)) r=mid;
    else l=mid;
  }
  cout<<a[r]<<endl;
  return 0;
}