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: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
using static System.Console;
using System.Text;
using System.IO;
using static System.Math;
namespace AtCoder
{
    class Program
    {
        static public long[] Sarray() { return ReadLine().Split().Select(long.Parse).ToArray(); }
        static public List<long> Slist() { return ReadLine().Split().Select(long.Parse).ToList(); }

        //static long Mod = (long)1e9 + 7;
        static List<long>[] tree;
        static long[] query;
        static bool[] cmp;
        static void Main(string[] args)
        {
            //SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
            var NK = Sarray();
            var N = NK[0];
            var K = NK[1];
            var xi = Sarray();
            var ans = long.MaxValue / 2;
            for (var l = 0; l < N - K + 1; ++l)
            {
                var r = l + K - 1;
                if (xi[l] < 0 && xi[r] <= 0)
                {
                    ans = Min(ans, Abs(xi[l]));
                }
                else if (xi[l] < 0 && 0 < xi[r]) 
                {
                    var t = Min(Abs(xi[l]) * 2 + xi[r], xi[r] * 2 + Abs(xi[l]));
                    ans = Min(ans, t);
                }
                else
                {
                    ans = Min(ans, xi[r]);
                }
            }
            WriteLine(ans);
            //Out.Flush();
        }
    }
}