import java.util.Scanner;

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

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