import java.util.Scanner;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int l = sc.nextInt() , k = sc.nextInt();
		if(l % (k*2) != 0) {
			System.out.print((l / (k* 2))*k);
		}else {
			System.out.print((l / (k* 2) - 1)*k);
		}
		
	}
}