import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} 

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} 

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} main {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int r = scanner.nextInt(); r scanner.nextInt() scanner.nextInt scanner int pi = 314159; pi 314159 System.out.println((double)(2*r*pi) / 100000) System.out.println System.out System System.out (double)(2*r*pi) / 100000 (double)(2*r*pi) 2*r*pi 2 r pi 100000 String[] args args public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} 

} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} 

} Main
import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		int r = scanner.nextInt();
		
		int pi = 314159;
		
		System.out.println((double)(2*r*pi) / 100000);
	} 

}