using System;
using System.Linq;

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

    }
}