Problem:
Score : 300 points

Problem Statement

There are N candles placed on a number line.
The i-th candle from the left is placed on coordinate x_i.
Here, x_1 < x_2 < ... < x_N holds.

Initially, no candles are burning.
Snuke decides to light K of the N candles.

Now, he is at coordinate 0.
He can move left and right along the line with speed 1.
He can also light a candle when he is at the same position as the candle, in negligible time.

Find the minimum time required to light K candles.

Constraints

1 \leq N \leq 10^5

1 \leq K \leq N

x_i is an integer.

|x_i| \leq 10^8

x_1 < x_2 < ... < x_N

Input

Input is given from Standard Input in the following format:

N K
x_1 x_2 ... x_N

Output

Print the minimum time required to light K candles.

Sample Input 1

5 3
-30 -10 10 20 50

Sample Output 1

40

He should move and light candles as follows:

Move from coordinate 0 to -10.

Light the second candle from the left.

Move from coordinate -10 to 10.

Light the third candle from the left.

Move from coordinate 10 to 20.

Light the fourth candle from the left.

Sample Input 2

3 2
10 20 30

Sample Output 2

20

Sample Input 3

1 1
0

Sample Output 3

0

There may be a candle placed at coordinate 0.

Sample Input 4

8 5
-9 -7 -4 -3 1 2 3 4

Sample Output 4

10

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

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

class Program
{
    static void Main(string[] args)
    {
    	var sc = new Scanner();
    	int n = sc.NextInt();
    	int k = sc.NextInt();
    	int[] x = new int[n];
    	var p = new List<int>(n);
    	var q = new List<int>(n);
    	
    	for (int i = 0; i < n; i++)
    	{
    	    int t = sc.NextInt();
    	    x[i] = t;
    	    if (t < 0) p.Add(-t);
    	    else q.Add(t);
    	}
    	p.Reverse();
    	
    	int ph = -1;
    	int qh = -1;
    	int c = 0;
    	int MAX = 2 * 100000000;
    	int min = MAX;
    	while (true)
    	{
    		if (ph < p.Count - 1 && (qh == q.Count - 1 || q[qh + 1] >= p[ph + 1])) ph++;
    		else if (qh < q.Count - 1 && (ph == p.Count - 1 || p[ph + 1] >= q[qh + 1])) qh++;
    		int sum = ph + 1 + qh + 1;
    		if (sum == k)
    		{
    			min = Math.Min(Math.Min(ph == -1 ? MAX : p[ph], qh == -1 ? MAX : q[qh]) * 2 + Math.Max(ph == -1 ? MAX : p[ph], qh == -1 ? MAX : q[qh]), min);
    		}
    		if (ph + 1 == k) min = Math.Min(p[ph], min);
    		if (qh + 1 == k) min = Math.Min(q[qh], min);
    		if (ph == p.Count - 1 && qh == q.Count - 1) break;
    	}
    	Console.WriteLine(min);
    }
}

class Scanner
{
    int index = -1;
    string[] buf;

    public Scanner()
    {
    }

    public int NextInt()
    {
        CheckNext();
        return int.Parse(buf[index++]);
    }
        
    public long NextLong()
    {
    	CheckNext();
    	return long.Parse(buf[index++]);
    }
    
    public double NextDouble()
    {
    	CheckNext();
    	return double.Parse(buf[index++]);
    }

    public string NextString()
    {
        CheckNext();
        return buf[index++];
    }

    private void CheckNext()
    {
        if (index >= 0 && index < buf.Length) return;

        buf = Console.ReadLine().Split(' ');
        index = 0;
    }
}