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;
		while(true){
			if(L>K*2){
				ans+=K;
				L-=K*2;
			}
			else{
				break;
			}
		}
		
		System.out.println(ans);
	}}