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

public class No0057 {
	public static void main(String[] args){
		try(BufferedReader br = new BufferedReader(new InputStreamReader(System.in))){
			System.out.println(Long.parseLong(br.readLine()) * 3.5);
		}catch(IOException e){
			e.printStackTrace();
		}
	}
}