import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long l = sc.nextLong();
		int k = sc.nextInt();
		sc.close();

		int eat = 0;
		int yuu = 0;
		while(eat < l){
			if(l <= eat + (k*2)) {
				break;
			}
			eat += k*2;
			yuu += k;
		}
		System.out.println(yuu);
	}
}