import java.util.Scanner;
class Wandbox
{
    public static void main(String[] args)
    {
    	Scanner sc = new Scanner(System.in);
    	
    	int l = sc.nextInt();
    	int k = sc.nextInt();
    	int ans = 0;
    	l /= 2;
    	
    	while (l - k > 0) {
    		l -= k;
    		ans += k;
    	} 
    	
    	System.out.println(ans);
    }
}