import java.util.Scanner;
/**
 * Write a description of class Circumfrence here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
public class Circumfrence
{
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        
        int num = sc.nextInt();
        
        System.out.println(3.14 * 2 * num);
    }
}

