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 = L/(2*K);
		if(L == K*count*2){
			System.out.println(K*(count-1));
		}
		else{
			System.out.println(K*count);
		}
	}
}