import java.util.Scanner;

public class yukicoder_63 {
	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int l = stdIn.nextInt();
		int k = stdIn.nextInt();
		
		int count = 0;
		
		while (l >= 2 * k + 1) {
			l = l - 2 * k;
			count++;
		}
		
		System.out.println(k * count);
	}
}