package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
)

func main() {
	r := bufio.NewReader(os.Stdin)
	var n, k int
	if _, err := fmt.Fscan(r, &n, &k); err != nil {
		return
	}
	f := make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Fscan(r, &f[i])
	}
	sort.Ints(f)
	ans := 0
	for i := n - 1; i >= 0; i -= k {
		ans += 2 * (f[i] - 1)
	}
	fmt.Println(ans)
}