Problem:
Score : 300 points

Problem Statement

There are 3N participants in AtCoder Group Contest.
The strength of the i-th participant is represented by an integer a_i.
They will form N teams, each consisting of three participants.
No participant may belong to multiple teams.

The strength of a team is defined as the second largest strength among its members.
For example, a team of participants of strength 1, 5, 2 has a strength 2, and a team of three participants of strength 3, 2, 3 has a strength 3.

Find the maximum possible sum of the strengths of N teams.

Constraints

1 ≤ N ≤ 10^5

1 ≤ a_i ≤ 10^{9}

a_i are integers.

Input

Input is given from Standard Input in the following format:

N
a_1 a_2 ... a_{3N}

Output

Print the answer.

Sample Input 1

2
5 2 8 5 1 5

Sample Output 1

10

The following is one formation of teams that maximizes the sum of the strengths of teams:

Team 1: consists of the first, fourth and fifth participants.

Team 2: consists of the second, third and sixth participants.

Sample Input 2

10
1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000 1000000000

Sample Output 2

10000000000

The sum of the strengths can be quite large.

Solution in Go (1.6) (judge verdict: Time Limit Exceeded)

package main

import(
      "fmt"
      "sort"
)

type Interface interface {
  Len() int
  Less(i, j int) bool
  Swap(i, j int)
}

func main(){
  var n int
  ans := 0
  fmt.Scan(&n)
  var a []int
  for i:=0; i<n*3; i++ {
    var tmp int
    fmt.Scan(&tmp)
    a = append(a, tmp)
  }
  sort.Ints(a)
  cnt := 0
  for i:=3*n-2; i>=0; i-=2 {
    if cnt == n {
      break;
    }
    ans+=a[i]
    cnt++;
  }
  fmt.Println(ans)
}

//====  ↓↓↓ template functions ↓↓↓  ======================

func min(a, b int64) int64{
  if a < b {
    return a
  }
  return b
}

func max(a, b int64) int64{
  if a > b {
    return a
  }
  return b
}

func abs(x int64) int64{
  if x < 0 {
    return -x
  }
  return x
}

func gcd(a, b int64) int64{
  if b == 0 {
    return a
  }else{
    return gcd(b, a%b)
  }
}

func lcm(a, b int64) int64{
  return a*b/gcd(a, b);
}