import java.util.*;
public class Kyosaya{
	public static void main(String args[])
	{
		Scanner s=new Scanner(System.in);
		int pocky = s.nextInt();
		int gnaw = s.nextInt();
		if(gnaw*2>= pocky){
			System.out.println(0);
		}else{
			int result = pocky-(pocky-(pocky/gnaw/2)*gnaw);
				if(result*2 == pocky){
					result= result - gnaw;
				}
		System.out.println(result);
		}
	}
}