import java.io.*;
import java.util.*;
public class Main
{
  public static void main(String args[])throws IOException
  {
    InputStreamReader read = new InputStreamReader(System.in);
    BufferedReader in = new BufferedReader(read);
    double r=Double.parseDouble(in.readLine());
    double circum=(double)(2*(Math.PI)*r);
    System.out.println(circum);
  }
}
/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