#include <iostream>
#include <stdio.h>
#include <string.h>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <algorithm>
#include <numeric>
#include <queue>
#include <cassert>
#include <cmath>
#include <bitset>
using namespace std;

int main() {
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  vector<pair<int, int>> b;
  for (int i = 1; i < n; i++) b.push_back({a[i], i});
  sort(b.begin(), b.end());
  //for (int i = 0; i < n-1; i++) cout << b[i].first << " " << b[i].second << endl;
  int ok = n-1;
  int ng = -1;
  while (ng+1 < ok) {
    int m = (ok+ng)/2;
    //printf("ng = %d, ok = %d, m = %d\n", ng, ok, m);
    vector<int> c;
    for (int i = 0; i < n-1; i++) if (i != m) c.push_back(b[i].first);
    vector<int> sums; 
    sums.push_back(a[0] + b[m].first);
    for (int i = 0; i < n-2; i += 2) sums.push_back(c[i] + c[i+1]);
    sort(sums.begin(), sums.end());
    //printf("sums : "); for (auto s : sums) printf("%d ", s); printf("\n");
    int r = sums.end() - upper_bound(sums.begin(), sums.end(), a[0] + b[m].first);
    //printf("r = %d\n", r);
    //printf("r < k = %d < %d\n", r, k);
    if (r < k) ok = m;
    else ng = m;
  }
  if (ok == n-1) cout << -1 << endl;
  else cout << b[ok].first << endl;
}