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.Collections.Generic;
using System.Linq;

namespace ABC107C
{
    class Program
    {
        static void Main(string[] args)
        {
            var dg = Console.ReadLine().Split(' ').Select(v => int.Parse(v)).ToArray();
            var n = dg[0];
            var k = dg[1] - 1;

            var x = Console.ReadLine().Split(' ').Select(v => double.Parse(v)).ToArray();

            var min = double.MaxValue;
            for (int i = 0; i < (n-k); ++i)
            {
                var start = x[i];
                var end = x[i + k];

                //Console.Write(start);
                //Console.Write(" ");
                //Console.WriteLine(end);
                //Console.WriteLine("---");

                double res = calc(start, end);

                if (res < min)
                {
                    min = res;
                }

                if (start >= 0)
                {
                    break;
                }
            }

            Console.WriteLine(min);
        }

        static double calc(double start, double end)
        {
            if (start < 0)
            {
                if (end < 0)
                {
                    return Math.Abs(start);
                }

                return getMin(start, end);
            }

            return end;
        }

        static double getMin(double start, double end)
        {
            var a = Math.Abs(end) + Math.Abs(start) * 2;
            var b = Math.Abs(end) * 2 + Math.Abs(start);

            //Console.Write(a);
            //Console.Write(" ");
            //Console.WriteLine(b);
            //Console.WriteLine("---");
            
            return a < b ? a : b;
        }
    }
}