import java.lang.*;
import java.util.*;
//codechef mein public nahi aata hai class se pehle
//or agr public lagana h to Class name Main hoga
//par codeforces men public class xyz aata hi aata he mtlb public aata hi h
public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
} import java.lang.*; lang.* java import java.util.*; util.* java public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
} Main public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    } main {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    } final Scanner s = new Scanner(System.in); s new Scanner(System.in) final int n = s.nextInt(); n s.nextInt() s.nextInt s double pi=3.1415926535; pi 3.1415926535 pi=pi*2 pi pi*2 pi 2 System.out.println(pi*n) System.out.println System.out System System.out pi*n pi n String args[] args public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
} public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
} Main
import java.lang.*;
import java.util.*;
//codechef mein public nahi aata hai class se pehle
//or agr public lagana h to Class name Main hoga
//par codeforces men public class xyz aata hi aata he mtlb public aata hi h
public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
}