import java.util.Scanner;
 
public class Main {
  
  private static double p = 2 * Math.PI;
 
    public static void main(String[] args) {
      Scanner in = new Scanner(System.in);
      int r = in.nextInt();
        
      in.close();
      
      System.out.println(p * r);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz