import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	} main {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) System.out.print(2*3.14159265*sc.nextInt()) System.out.print System.out System System.out 2*3.14159265*sc.nextInt() 2*3.14159265*sc.nextInt() 2 3.14159265 sc.nextInt() sc.nextInt sc String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		System.out.print(2*3.14159265*sc.nextInt());
	}
}