import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

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

} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

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

} Main public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

		System.out.println(Math.PI * n * 2);
		stdIn.close();
	} main {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

		System.out.println(Math.PI * n * 2);
		stdIn.close();
	} Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int n = stdIn.nextInt(); n stdIn.nextInt() stdIn.nextInt stdIn System.out.println(Math.PI * n * 2) System.out.println System.out System System.out Math.PI * n * 2 Math.PI * n * 2 Math.PI Math Math.PI n 2 stdIn.close() stdIn.close stdIn String[] args args public class Main {
	public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

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

} public class Main {
	public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

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

} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();

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

}