import java.util.*;
 
public class Main{
    final static long mod = 1000000007;
    //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    }
} import java.util.*; util.* java public class Main{
    final static long mod = 1000000007;
    //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    }
} Main final static long mod = 1000000007; mod 1000000007 //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    } main {
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double r = sc.nextDouble(); r sc.nextDouble() sc.nextDouble sc System.out.println(r * 2 * Math.PI) System.out.println System.out System System.out r * 2 * Math.PI r * 2 * Math.PI r 2 Math.PI Math Math.PI String[] args args public class Main{
    final static long mod = 1000000007;
    //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    }
} public class Main{
    final static long mod = 1000000007;
    //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    }
} Main
import java.util.*;
 
public class Main{
    final static long mod = 1000000007;
    //
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double r = sc.nextDouble();
        System.out.println(r * 2 * Math.PI);
    }
}