Problem:
Score: 400 points

Problem Statement

We have N integers A_1, A_2, ..., A_N.

There are \frac{N(N-1)}{2} ways to choose two of them and form a pair. If we compute the product of each of those pairs and sort the results in ascending order, what will be the K-th number in that list?

Constraints

All values in input are integers.

2 \leq N \leq 2 \times 10^5

1 \leq K \leq \frac{N(N-1)}{2}

-10^9 \leq A_i \leq 10^9\ (1 \leq i \leq N)

Input

Input is given from Standard Input in the following format:

N K
A_1 A_2 \dots A_N

Output

Print the answer.

Sample Input 1

4 3
3 3 -4 -2

Sample Output 1

-6

There are six ways to form a pair. The products of those pairs are 9, -12, -6, -12, -6, 8.

Sorting those numbers in ascending order, we have -12, -12, -6, -6, 8, 9. The third number in this list is -6.

Sample Input 2

10 40
5 4 3 2 -1 0 0 0 0 0

Sample Output 2

6

Sample Input 3

30 413
-170202098 -268409015 537203564 983211703 21608710 -443999067 -937727165 -97596546 -372334013 398994917 -972141167 798607104 -949068442 -959948616 37909651 0 886627544 -20098238 0 -948955241 0 -214720580 277222296 -18897162 834475626 0 -425610555 110117526 663621752 0

Sample Output 3

448283280358331064

Solution in C# (Mono 4.6.2.0) (judge verdict: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CSharpConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
          Pairs();
        }
        static void Pairs()
        {
            int[] nk = Array.ConvertAll<string,int>( Console.ReadLine().Split(' '),int.Parse);
            int[] a_n = Array.ConvertAll<string, int>(Console.ReadLine().Split(' '), int.Parse);

            int N = nk[0];
            int K = nk[1];
            List<int> seki = new List<int>();
            for(int i = 0; i < N; i++)
            {
                for (int j = i + 1; j < N; j++)
                {
                    seki.Add(a_n[i] * a_n[j]);
                }
            }
            int[] order = seki.OrderBy((x) => x).ToArray();
            Console.WriteLine(order[K]);

        }      
    }
}