import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int R = sc.nextInt();
		sc.close();

		double ret = getProcess(R);

		System.out.println(ret);
	}

	public static double getProcess(int R){
		double ret = 2 * Math.PI * R ;

		return ret;
	}


}
/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