import java.util.*;
public class Main {// Main
    public static void main(String[] args){
        	Scanner sc=new Scanner (System.in);
		
		double r=sc.nextDouble();
		double circum=2*Math.PI*r;
		System.out.println(String.format("%.20f", 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