import java.util.Scanner;

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