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

public class Main {

	public static void main(String[] args) {
	      BufferedReader stdReader =new BufferedReader(new InputStreamReader(System.in));
	      String line = "";
	      try {
	    	  String[] temps = stdReader.readLine().split(" ");
	    	  int L = Integer.parseInt(temps[0]);
	    	  int K = Integer.parseInt(temps[1]);
	    	  if(L%(2*K) != 0){
	    		  System.out.println(L/(2*K)*K);
	    	  }else{
	    		  System.out.println((L/(2*K)-1)*K);
	    	  } 
	      } catch (IOException e) {
			e.printStackTrace();
	      }
	}
}