import java.util.Arrays;
import java.util.Scanner;
public class length{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
	    int a = sc.nextInt();
	    int b = sc.nextInt();
        int wari = a / (b * 2);
        int dasi = wari * b;
        int ate = a / 2;
        if(ate == dasi){
            System.out.println(dasi);
        }else{
            System.out.println("0");
        }
	    sc.close();
    }
}