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

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String[] inputs = reader.readLine().split(" ");
        long L = Long.parseLong(inputs[0]);
        int K = Integer.parseInt(inputs[1]);

        int yuu = 0;
        while (2*K < L) {
            yuu += K;
            L -= 2 * K;
        }

        System.out.println(yuu);

    }

}