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

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

	      try {
		      double N =Long.parseLong(stdReader.readLine());
		      System.out.println(3.5*N);
	    	  } catch (IOException e) {
			e.printStackTrace();
	      }
	}	
}