import java.util.*;
 
class Main{
  public static void main(String[] args) {
    Scanner reader= new Scanner(System.in);
    double r =reader.nextDouble();
    System.out.println(2*r*3.14159265);
 
 
 
 
  }
} import java.util.*; util.* java class Main{
  public static void main(String[] args) {
    Scanner reader= new Scanner(System.in);
    double r =reader.nextDouble();
    System.out.println(2*r*3.14159265);
 
 
 
 
  }
} Main public static void main(String[] args) {
    Scanner reader= new Scanner(System.in);
    double r =reader.nextDouble();
    System.out.println(2*r*3.14159265);
 
 
 
 
  } main {
    Scanner reader= new Scanner(System.in);
    double r =reader.nextDouble();
    System.out.println(2*r*3.14159265);
 
 
 
 
  } Scanner reader= new Scanner(System.in); reader new Scanner(System.in) double r =reader.nextDouble(); r reader.nextDouble() reader.nextDouble reader System.out.println(2*r*3.14159265) System.out.println System.out System System.out 2*r*3.14159265 2*r*3.14159265 2 r 3.14159265 String[] args args
import java.util.*;
 
class Main{
  public static void main(String[] args) {
    Scanner reader= new Scanner(System.in);
    double r =reader.nextDouble();
    System.out.println(2*r*3.14159265);
 
 
 
 
  }
}
