import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	}
} Main public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	} 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 String[] args args public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	}
} public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		System.out.println(2*N*Math.PI);
	}
} Main
import java.util.Scanner;

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