import java.util.Scanner;

class Test {
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		int l = in.nextInt();
		int k = in.nextInt();

		int temp = 0;
		for(int i=l; i>0; i--) {
			i -= k * 2;
			temp += k;
		}

		System.out.println(temp);
	}
}