import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int len = sc.nextInt();
        int bite = sc.nextInt();
        int eat = 0;
        
        while((len - bite * 2) > 0){
            len -= bite * 2;
            eat += bite;
        }
        System.out.println(eat);
    }
}