import java.util.Scanner;

public class PockyGame {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int L = s.nextInt(), K = s.nextInt();
		s.close();
		if(L % (2 * K) == 0){
			System.out.println((L/(2*K)-1) *K);
		}else{
			System.out.println((L/(2*K)) * K);
		}

	}

}