import java.util.Scanner;
//import java.util.Arrays;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	}
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	} main {
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) System.out.println(sc.nextInt() * 6.28) System.out.println System.out System System.out sc.nextInt() * 6.28 sc.nextInt() sc.nextInt sc 6.28 String[] args args public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	}
} public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	}
} Main
import java.util.Scanner;
//import java.util.Arrays;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		System.out.println(sc.nextInt() * 6.28);
	}
}
