import java.util.Scanner;

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