import java.util.*;

public class Main {
  public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int a = sc.nextInt();
          double b = 2 * a * 3.14;
          System.out.println(b);
        }
      }
/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