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;
		
		if(L<=K || L==K*2)	yuu=0;
		else if(nokori<=K*2){
			yuu=K;
		}else{
			while(nokori>K*2){
				nokori = L-K*2;
				yuu += K;
				L=nokori;
			}
		}
		
        System.out.println(yuu);
    }
}