import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int l = sc.nextInt();
		int k = sc.nextInt();
		int ans = 0;
		while(l > 2 * k) {
			ans += k;
			l -= 2 * k;
		}
		System.out.println(ans);
 	}
}