import java.util.Scanner;
public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
} Main public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    } main {
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    } Scanner v = new Scanner(System.in); v new Scanner(System.in) int x = v.nextInt(); x v.nextInt() v.nextInt v System.out.print(obseg(x)) System.out.print System.out System System.out obseg(x) obseg x String[] args args public static double obseg(int x){
	return 2*3.1415*x;
  } obseg {
	return 2*3.1415*x;
  } return 2*3.1415*x; 2*3.1415*x 2*3.1415*x 2 3.1415 x int x x public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
} public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
} Main
import java.util.Scanner;
public class Main{
	public static void main(String[] args){
      Scanner v = new Scanner(System.in);
      int x = v.nextInt();
      System.out.print(obseg(x));
    }
  public static double obseg(int x){
	return 2*3.1415*x;
  }
}
