package me.yukicoder.lv1;
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))) {
			String[] str = br.readLine().split(" ");
			long d = Long.parseLong(str[0]);
			System.out.println(d * 3.5);

		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}