#include <bits/stdc++.h>
using namespace std;
#define rep(i, m, n) for (int i = m; i < n; ++i)
#define rem(i, m, n) for (int i = m; i >= n; --i)
typedef long long ll;

int main(){
  int N, M;
  cin >> N >> M;
  vector<int> a(N);
  rep(i, 0, N) cin >> a[i];
  sort(a.begin() + 1, a.end());
  int bl = 0;
  int br = N;
  int mid;
  while(abs(bl - br) > 1) {
    mid = (bl + br) / 2;
    int cnt = 0;
    int l = 1;
    int r = N - 1;
    while(l < r){
      if(l == mid) l++;
      else if(r == mid) r--;
      else if(a[l] + a[r] > a[0] + a[mid]) cnt++, l++, r--;
      else l++;
    }
    if(cnt < M) br = mid;
    else bl = mid;
  }
  cout << (br == N ? -1 : a[br]) << endl;
  return 0;
}