import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	// 期待値の線形性 E[X + Y] = E[X] + E[Y] から自明
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		
		System.out.printf("%.8f\n", 3.5 * N);
	}
	
}