import java.lang.*;
import java.util.*;
public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    }
} import java.lang.*; lang.* java import java.util.*; util.* java public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    }
} Main ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    } main {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    } final Scanner s = new Scanner(System.in); s new Scanner(System.in) int r=s.nextInt(); r s.nextInt() s.nextInt s double ans=3.14159265; ans 3.14159265 ans=ans*2.0 ans ans*2.0 ans 2.0 System.out.println(ans*r) System.out.println System.out System System.out ans*r ans r final String args[] args public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    }
} public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    }
} Main
import java.lang.*;
import java.util.*;
public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*r);
    }
}