import java.math.*;
import java.util.*;
public class Main
{
    public static void main(String[] args)
    {
    	BigDecimal pi=new BigDecimal("3.14159265359");
    	Scanner sc = new Scanner(System.in);
    	BigDecimal m = sc.nextBigDecimal();
    	System.out.println(m.multiply(pi).multiply(new BigDecimal(2)));
    	
    }
}
/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