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

const int MAXN = int(1e5);

int N, M;
int A[MAXN];

int main() {
  ios::sync_with_stdio(false);
  cin >> N >> M;
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
  }
  vector<int> dsc;
  for (int i = 0; i < N; ++i) {
    dsc.emplace_back(A[i]);
  }
  sort(dsc.begin(), dsc.end());
  dsc.erase(unique(dsc.begin(), dsc.end()), dsc.end());
  int lb = 0, ub = int(dsc.size()) - 1;
  int ans = -1;
  while (lb <= ub) {
    int mb = lb + ub >> 1;
    vector<int> z;
    bool flag = false;
    int us = A[0] + dsc[mb];
    for (int i = 1; i < N; ++i) {
      if (not flag and A[i] == dsc[mb]) {
        flag = true;
        continue;
      }
      z.emplace_back(A[i]);
    }
    sort(z.begin(), z.end());
    int lp = 0, rp = int(z.size()) - 1;
    int cnt = 0;
    while (lp < rp) {
      while (lp < rp and z[lp] + z[rp] <= us) ++lp;
      if (rp - lp + 1 < 2) break;
      ++cnt;
      ++lp;
      --rp;
    }
    if (flag and cnt < M) {
      ub = mb - 1;
      ans = dsc[mb];
    } else {
      lb = mb + 1;
    }
  }
  cout << ans << endl;
  return 0;
}