import java.util.Scanner;

public class Main57 {
	public static void main(String[] args) {
		Main57 p = new Main57();
	}

	public Main57() {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		System.out.println(n*3.5);
		
	}

	public void solve() {
	}

}