import java.util.*;

public class ok{
	public static void main(String[] args) {
		Scanner sc=new Scanner (System.in);
		int L=sc.nextInt();
		int K=sc.nextInt();
		int ans=0;
		if(L%(K*2)!=0)ans=L/(K*2);
		else ans=L/(K*2)-1;
		ans*=K;
		
		System.out.println(ans);
	}}