import java.util.*;

public class Main {
	public static void main(String[] args){

		Scanner scan01 = new Scanner(System.in);
		int num = scan01.nextInt();
		scan01.close();

		int mult = num*2;

		double circle= mult * 3.14;

		System.out.println(circle);
	}
}

/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