import java.lang.*;
import java.util.*;
public class Main 
{
    ///atcoder
    public static void main(final String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        int r=s.nextInt();
        double ans=3.14159265;
        ans=ans*2.0;
        System.out.println(ans*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