import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	}
} Main public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	} main {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) System.out.println(2 * sc.nextInt() * Math.PI) System.out.println System.out System System.out 2 * sc.nextInt() * Math.PI 2 * sc.nextInt() * Math.PI 2 sc.nextInt() sc.nextInt sc Math.PI Math Math.PI String[] args args public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	}
} public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		System.out.println(2 * sc.nextInt() * Math.PI);
	}
}