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

public class Test{
	public static void main(String[] args) {
		try{
			getAns();
		}
		catch(IOException e){
			e.getStackTrace();
		}
		catch(NumberFormatException e){
			e.getStackTrace();
		}
	}
	
	public static void getAns() throws IOException, NumberFormatException{
		double ans = 0;
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		double N = Double.parseDouble(br.readLine());
		ans = N * 3.5;
		System.out.println(ans);
	}
}