public class Sample {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int R = sc.nextInt();
//        int b = sc.nextInt();
//        String s = sc.next();
//        if ((a * b) % 2 == 0)
//            System.out.println("Even");
//        else
//            System.out.println("Odd");
        System.out.println(Math.PI * R);
        
	}
}