import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int L = sc.nextInt();
        int K = sc.nextInt();
int r =L/(2*K);

if(L%(2*K)==0){
r=r-1;
}
int a=r*K;
        System.out.println(a);
    }
}