import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt(), k = sc.nextInt(), tmp;
		tmp = n/(k*2);
		if(n%(k*2) == 0){
			tmp--;
		}
		System.out.println(k*tmp);
	}
}