package main
import "fmt"

func main() {
	var pocky, bite int
	fmt.Scanf("%d %d", &pocky, &bite)

	fmt.Println(solve(pocky, bite))
}

func solve(pocky int, bite int) int {
	count := pocky / (bite * 2)
	mod := pocky % (bite * 2)

	if mod == 0 {
		count = count - 1
	}
	return count * bite
}