import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int L=sc.nextInt(); //90
        int K=sc.nextInt(); //20 *2
        int result=0;

        for(int i = K * 2; i < L; ){
            if(K * 2 < L){
                result+=K;
            }
            i+=K*2;
        }
        System.out.println(result);

            sc.close();
		}
    }