/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] line = br.readLine().split(" ");
        int L = Integer.parseInt(line[0]);
        int K = Integer.parseInt(line[1]);
        
        int ans = 0;
        if(L%(2*K) == 0){
            ans = K*((L-1)/(2*K));
        }else{
            ans = K*((L)/(2*K));
        }
        
        System.out.println(ans);
    }
}