import java.util.Scanner;

class Main {
  public static void main(String[] args) {

    Scanner scan = new Scanner(System.in);
    String str = scan.nextLine();
    double res = Integer.parseInt(str);
    res = res * 2 * Math.PI;
    System.out.println(res);
  }

}

/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