/* -*- coding: utf-8 -*-
 *
 * 507.cc: No.507 ゲーム大会(チーム決め) - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

/* global variables */

int as[MAX_N];

/* subroutines */

int check(int n, int k, int a0) {
  int sum = a0 + as[k];
  int cnt = 0;
  for (int i = 0, j = n - 1; i < j; j--) {
    if (j == k) continue;
    while (i < j && (i == k || as[i] + as[j] <= sum)) i++;
    if (i >= j) break;
    cnt++;
    i++;
  }
  return cnt;
}

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  n--;

  int a0;
  scanf("%d", &a0);
  for (int i = 0; i < n; i++) scanf("%d", &as[i]);
  sort(as, as + n);

  int k0 = -1, k1 = n;
  while (k0 + 1 < k1) {
    int k = (k0 + k1) / 2;
    int c = check(n, k, a0);
    //printf("k0=%d,k1=%d,k=%d,c=%d\n", k0, k1, k, c);
    if (c < m) k1 = k;
    else k0 = k;
  }

  printf("%d\n", (k1 >= n) ? -1 : as[k1]);
  return 0;
}