import java.util.*;

public class Main {

  static Scanner scan = new Scanner(System.in);

  public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  }
} import java.util.*; util.* java public class Main {

  static Scanner scan = new Scanner(System.in);

  public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  }
} Main static Scanner scan = new Scanner(System.in); scan new Scanner(System.in) public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  } main {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  } int t = scan.nextInt(); t scan.nextInt() scan.nextInt scan System.out.print(2 * t * 3.14159) System.out.print System.out System System.out 2 * t * 3.14159 2 * t * 3.14159 2 t 3.14159 String[] args args public class Main {

  static Scanner scan = new Scanner(System.in);

  public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  }
} public class Main {

  static Scanner scan = new Scanner(System.in);

  public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  }
} Main
import java.util.*;

public class Main {

  static Scanner scan = new Scanner(System.in);

  public static void main(String[] args) {
    int t = scan.nextInt();
    System.out.print(2 * t * 3.14159);
  }
}