import java.util.*;
import java.math.*;
import java.io.*;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int L = sc.nextInt(); 
		int K = sc.nextInt();
		System.out.println(((L / (2 * K) - (L % (2 * K) == 0?1:0)) * K));
	}
}