import java.util.*;
class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    pl(2*Math.PI*a+"");
  }

  public static void pr(String str){
    System.out.print(str);
  }
  public static void pl(String str){
    System.out.println(str);
  }
} import java.util.*; util.* java class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    pl(2*Math.PI*a+"");
  }

  public static void pr(String str){
    System.out.print(str);
  }
  public static void pl(String str){
    System.out.println(str);
  }
} Main public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    pl(2*Math.PI*a+"");
  } main {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    pl(2*Math.PI*a+"");
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc pl(2*Math.PI*a+"") pl 2*Math.PI*a+"" 2*Math.PI*a 2*Math.PI*a 2 Math.PI Math Math.PI a "" String args[] args public static void pr(String str){
    System.out.print(str);
  } pr {
    System.out.print(str);
  } System.out.print(str) System.out.print System.out System System.out str String str str public static void pl(String str){
    System.out.println(str);
  } pl {
    System.out.println(str);
  } System.out.println(str) System.out.println System.out System System.out str String str str
import java.util.*;
class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    pl(2*Math.PI*a+"");
  }

  public static void pr(String str){
    System.out.print(str);
  }
  public static void pl(String str){
    System.out.println(str);
  }
}
