import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
       int l = koko.nextInt();
       int k = koko.nextInt();
       int over = l%(2*k);
       if(over!=0){
          System.out.println((l-over)/2); 
       }else if(over==0){
           System.out.println((l-2*k)/2);
       }
       
    }
}