Library

You work at a library and want to put a number of books back on the shelves. The shelves are placed along the $x$-axis. Each book's target shelf is given as an $x$-coordinate between $-1\,000$ and $1\,000$, along with the maximum number of books you can carry at once. Find the shortest distance you must walk to return all books. The books start at position $0$. You do not need to walk back after returning the last book.

The first line contains two integers $N$ and $K$, where $1 \le N \le 100$ is the number of books to return and $1 \le K \le 100$ is the number of books you can carry at once. Each of the next $N$ lines contains one integer, the $x$-coordinate of the shelf where that book goes.

Print one integer on a single line: the minimum distance you must walk to return all the books.

For example, you can start by carrying the books for shelves 3 and 4. Then fetch the book for shelf 1, and finally take the book for shelf -4.