import java.util.Arrays;
import java.util.Scanner;
public class length{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
	    int a = sc.nextInt();
	    int b = sc.nextInt();
	    int c = 0;
        while(a > b*2){
			a -= b*2;
			c+=1;
		}
		System.out.println(b * c);
		sc.close();
    }
}