import java.util.Scanner;
public class Main {
	  public static void main(String args[]){
	 Scanner scanner = new Scanner(System.in);

double a=scanner.nextDouble();
double c=a*2*3.14;
	System.out.println(c);
}
	  }
/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