import java.util.Scanner;

public class Yukicoder63 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int L = sc.nextInt();
		int K = sc.nextInt();
		int res = 0;
		
		while(L > 0) {
			L -= K * 2;
			if (L > 0) {
				res += K;
			}
		}
		System.out.println(res);
	}

}