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

class Main63 {
	
	public static void main (String[] args) throws IOException {
		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]);
		double d = (double)l / (k * 2), sub = d - (int)d;
		
		System.out.println(sub == 0 ? ((int)d - 1) * k : (int)d * k);
		
	}
}