#include<bits/stdc++.h>
using namespace std;
#define int long long

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n,m; cin >> n >> m;
  vector<int> a(n);
  for(int i = 0; i < n; i++){
    cin >> a[i];
  }

  sort(a.begin()+1,a.end());
  if(n == m*2){
    cout << a[1] << endl;
    return 0;
  }
  vector<int> rival(m*2);
  for(int i = 0; i < n; i++){
    if(i >= m*2) break;
    rival[i] = a[n-1 - i];
  }
  int need = rival[0] + rival[m*2-1] - a[0];
  for(int i = 1; i < n-m*2; i++){
    if(a[i] >= need){
      cout << a[i] << endl;
      return 0;
    }
  }
  cout << -1 << endl;

  return 0;
}