import java.util.Scanner;
public class No63 {
	public static void main(String[]args) {
		Scanner sc = new Scanner(System.in);

		int length=sc.nextInt();
		int K = sc.nextInt();

		int cnt=0;

		do {
			length = length-2*K;

			if(length==0)	break;

			cnt += K;

		}while(length>K);

		System.out.println(cnt);

	}

}