import java.util.Scanner;

public class pre {
    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);
		
		int l = s.nextInt(), k = s.nextInt();
		int ans =0;
		while((l - 2*k) >0){
			l -= 2 * k;
			ans += k;
		}
		System.out.println(ans);
    }
}