import java.util.*;

public class Yukicoder_63{
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		int L = stdIn.nextInt();
		int K = stdIn.nextInt();
		int count = 0;
		while(true){
			if(L<=2*K){
				System.out.println(count*K);
				break;
			}
			L -= 2*K;
			count++; 
		}
	}
}