import java.io.*;
import java.util.*;
import java.lang.*;
import java.math.*;
public class Main{

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int R=in.nextInt();
		in.close();
		double result=2*R*Math.PI;
		System.out.println(result);
	}

}

/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