import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		while(sc.hasNext()){
			long l=sc.nextLong();
			long k=sc.nextLong();
			long ans=0;
			while(true){
				if(l-2*k<=0) break;
				else{
					ans+=k;
					l-=2*k;
				}
			}
			System.out.println(ans);
		}
	}
}