import java.util.*;

public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int length = sc.nextInt();
		int a = sc.nextInt();
		int yu = 0;
		int ha = 0;
		while (true) {
			if (length-a*2<=0) {break;}
			length -= a*2;
			yu += a;
			ha += a;
		}
		System.out.println(yu);
	}
}