import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class No0063 {
	public static void main(String[] args) {
		try (BufferedReader br = 
				new BufferedReader(new InputStreamReader(System.in))) {
			String[] str = br.readLine().split(" ");
			int L = Integer.parseInt(str[0]);
			int K = Integer.parseInt(str[1]);
			int cnt;
			
			for(cnt = 0; L > (K * 2); cnt++){
				L -= K * 2;
			}
	        System.out.println(K * cnt);
	        
		} catch (IOException e) {
			e.printStackTrace();
		}
    }
}