import java.lang.*;
import java.util.*;
//codechef mein public nahi aata hai class se pehle
//or agr public lagana h to Class name Main hoga
//par codeforces men public class xyz aata hi aata he mtlb public aata hi h
public class Main 
{
    public static void main( String args[]) 
    {
        final Scanner s = new Scanner(System.in);
        final int n = s.nextInt();
        double pi=3.1415926535;
        pi=pi*2;
        System.out.println(pi*n);
    }
}
/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