import java.util.*;
public class Dice{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		double n,res;
		n = sc.nextInt();
		res = n * 3.5;
		if(n % 2 == 0){
			System.out.println((int)res);
		}else{
			System.out.println(res);
		}
	}
}