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();
        double ans=R*Math.PI*2;
        System.out.println(ans);
        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