fun main(args: Array<String>){
	var (a,b)=readLine()!!.split(" ").map(String::toInt)
	if (a / (b * 2) == 2 && a % (b * 2) == 0) {
        println(0)
    } else if (a % (b * 2) == 0) {
        println(((a / (b * 2)) - 1) * b)
    } else {
        println((a / (b * 2)) * b)
    }
}