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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

#define REP(i, start, count) for(int i=(start); i<(int)(count); ++i)
#define rep(i, count) REP(i, 0, count)
#define ALLOF(c) (c).begin(), (c).end()

typedef long long ll;
typedef unsigned long long ull;

int N, K;

int main(void) {
    cin >> N >> K;

    vector<int> x(N);
    int posbeg_i = -1;
    rep(i, N) {
        cin >> x[i];
        if (posbeg_i == -1 && x[i] >= 0) {
            posbeg_i = i;
        }
    }

    ll ans = 1000000000000000000LL;

    if (N == K == 1) {
        cout << abs(x[0]) << endl;
        return 0;
    }

    for(int l = 0; l < N - K; ++l) {
        int r = l + K - 1;
        ll ltor = abs(x[l]) + abs(x[r] - x[l]); 
        ll rtol = abs(x[r]) + abs(x[r] - x[l]);

        ans = min(ans, min(ltor, rtol));
    } 

    cout << ans << endl;
    return 0;
}