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

public class No57 {

	public static void main(String[] args) throws IOException{
		//ミリオンダイス
		System.out.println(Double.parseDouble(readStr()[0]) * 3.5);

	}
	
	public static String[] readStr() throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		ArrayList<String> list = new ArrayList<>();

		do {
			list.add(br.readLine());
		}while(br.ready());

		br.close();

		String[] text = new String[list.size()];
		list.toArray(text);

		return text;

	}

}