import java.util.Scanner;

public class Answer {
	public static void main(String str[]) {
        Scanner sc = new Scanner(System.in);

        int length = Integer.valueOf(sc.next());
        int oneTime = Integer.valueOf(sc.next());

        int i=0;
        while(oneTime*2 < length) {
        	length -= oneTime*2;
        	i++;
        }

        System.out.println(i*oneTime);
	}
}