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