import java.util.Scanner;

class Main {
	public static void main(String[] $){
		Scanner sc =new Scanner(System.in);
		int n = sc.nextInt();

		System.out.println(2 * n * Math.PI);

		sc.close();
	}
}
 import java.util.Scanner; Scanner java.util class Main {
	public static void main(String[] $){
		Scanner sc =new Scanner(System.in);
		int n = sc.nextInt();

		System.out.println(2 * n * Math.PI);

		sc.close();
	}
} Main public static void main(String[] $){
		Scanner sc =new Scanner(System.in);
		int n = sc.nextInt();

		System.out.println(2 * n * Math.PI);

		sc.close();
	} main {
		Scanner sc =new Scanner(System.in);
		int n = sc.nextInt();

		System.out.println(2 * n * Math.PI);

		sc.close();
	} Scanner sc =new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc System.out.println(2 * n * Math.PI) System.out.println System.out System System.out 2 * n * Math.PI 2 * n * Math.PI 2 n Math.PI Math Math.PI sc.close() sc.close sc String[] $ $
import java.util.Scanner;

class Main {
	public static void main(String[] $){
		Scanner sc =new Scanner(System.in);
		int n = sc.nextInt();

		System.out.println(2 * n * Math.PI);

		sc.close();
	}
}

