#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>

#include <functional>
#include <cassert>

typedef long long ll;
using namespace std;

#define debug(x) cerr << #x << " = " << (x) << endl;


#define mod 1000000007 //1e9+7(prime number)
#define INF 1000000000 //1e9
#define LLINF 2000000000000000000LL //2e18
#define SIZE 100010

int n,m,a[SIZE];

bool check(int p){
  int a2[SIZE];
  int t = 0;

  for(int i=1;i<n;i++){
    if(i == p) continue;
    a2[t++] = a[i];
  }

  int q = 0;
  int ee = 0;
  
  for(int i=t-1;i>=0;i--){
    while(q < i && a2[i] + a2[q] <= a[0] + a[p]) q++;

    if(q >= i) break;

    q++;
    ee++;
  }
  
  return m > ee;
}

int main(){

  scanf("%d%d",&n,&m);

  for(int i=0;i<n;i++){
    scanf("%d",a+i);
  }

  sort(a+1,a+n);

  int l = 1, r = n-1;

  if(check(n-1) == false){
    puts("-1");
    return 0;
  }
  
  while(l < r){
    int mid = (l+r)/2;

    if(check(mid)){
      r = mid;
    }else{
      l = mid+1;
    }
  }

  printf("%d\n",a[l]);
  
  return 0;
}