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

public class Main {
	public static void main(String[] args) {
		BufferedReader buff = new BufferedReader(new InputStreamReader(System.in));

		try {
			int num = Integer.parseInt(buff.readLine());
			System.out.println(3.5 * num);
		} catch (NumberFormatException e) {

		} catch (IOException e) {

		}
	}
}