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 Go (1.6) (judge verdict: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n := scanInt(sc)
	k := scanInt(sc)

	minNegIdx := -1
	xs := make([]int, n)
	for i := 0; i < n; i++ {
		xs[i] = scanInt(sc)
		if xs[i] < 0 {
			minNegIdx = i
		}
	}

	leftmost := int(math.Max(0, float64(minNegIdx-k)))
	rightmost := int(math.Min(float64(n-k), float64(minNegIdx+1)))

	minTime := math.MaxInt32
	for i := leftmost; i <= rightmost; i++ {
		var time int
		end := i + k - 1

		if end <= minNegIdx {
			time = abs(xs[i])
		} else if minNegIdx < i {
			time = xs[end]
		} else {
			if xs[i] < xs[end] {
				if xs[end] == 0 {
					time = abs(xs[i])
				} else {
					time = abs(xs[i])*2 + xs[end]
				}
			} else {
				time = abs(xs[i]) + xs[end]*2
			}
		}

		if time < minTime {
			minTime = time
		}
	}

	fmt.Println(minTime)
}

func abs(n int) int {
	if n < 0 {
		return -n
	}
	return n
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}