import java.util.Scanner;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int L = scan.nextInt();
        int K = scan.nextInt();
        if (L < 1||L > 1000000000||K < 1||K > 50) System.exit(1);

        System.out.println((L-1)/(2*K) * K);
    }
}