import java.util.*;
 

public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int y = sc.nextInt();
		int ans=0;
		while(x>y*2){
			x-=y*2;
			ans++;
		}
		System.out.println(ans*y);
	}
}