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

class Program
{
    static void Main(string[] args)
    {
        // 入力
        int[] nk = Console.ReadLine().Split().Select(int.Parse).ToArray();
        var n = nk[0];
        var k = nk[1];
        int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();

        var l = 0;
        var r = 0;
        // 正負が切り替わるポイントを見つける
        for (int i = 0; i < x.Length; i++)
        {
            if (x[i] > 0)
            {
                if (i >= k)
                {
                    r = i;
                    l = i - k;
                }
                else
                {
                    l = 0;
                    r = l + k - 1;
                }
                break;
            }
        }

        var fr = r;
        var ok = true;
        var ans = int.MaxValue;
        while (ok)
        {
            //l-r
            var lr = Math.Abs(x[l]) + Math.Abs(x[r] - x[l]);
            //r-l
            var rl = Math.Abs(x[r]) + Math.Abs(x[r] - x[l]);
            var tmp = Math.Min(lr,rl);
            ans = Math.Min(ans, tmp);
            l++;
            r++;
            if (r == x.Length || l == fr)
            {
                ok = false;
            }
        }


        Console.WriteLine(ans);

    }

}