import java.util.*;

class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		
		int l = sc.nextInt(); int k = sc.nextInt();
		if (l%(2*k) == 0) {
			System.out.println((l/(2*k)-1)*k);
		}
		else {
			System.out.println(l/(2*k)*k);
		}
		
		
		
	}
}