using System;
using System.Linq;

public class yukicoder{
    public static void Main(){
        string[] input = Console.ReadLine().Trim().Split(' ');
        int length = int.Parse(input[0]);
        int eat = int.Parse(input[1]);
        
        int answer = ((length - 1) / (eat * 2)) * eat;
        
        Console.WriteLine(answer);

    }
}