import java.util.*;
import java.lang.Math;
public class Main{
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int r=sc.nextInt();
        System.out.println(2.0 * Math.PI * (double)r);
      sc.close();

    }
}

/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