import java.util.*;

public class Main {
  public static void main(String[] args ) throws Exception {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int x = sc.nextInt();
    
    if(N/x/2==1 && N%(x*2)==0){
        System.out.println(0);
    }else if(N%(x*2)==0){
        int j=N/(x*2);
        int y=(j-1)*x;
        System.out.println(y);
    }else{
        int y=(N/x/2)*x;
        System.out.println(y);
    }

  }
}