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

int main() {
  int n,k;
  cin >> n >> k;
  int a[n];
  for(int i=0; i<n; i++) cin >> a[i];
  sort(a+1,a+n,greater<int>());
  int l=0,r=n;
  while(l+1!=r) {
    int m=(l+r)/2;
    int l2,r2=n-1,c=0;
    if(r2==m) r2--;
    for(l2=1; l2<r2; l2++,r2--) {
      if(l2==m) continue;
      while(l2<r2&&a[l2]+a[r2]<=a[0]+a[m]) {
        r2--;
        if(r2==m) r2--;
      }
      if(l2>=r2) break;
      c++;
    }
    if(c>=k) r=m;
    else l=m;
  }
  cout << (l?a[l]:-1) << endl;
  return 0;
}