#include"bits/stdc++.h"
#include<boost/multi_array.hpp>
#include<boost/optional.hpp>
#include<boost/range/irange.hpp>
#include<boost/range/algorithm.hpp>
#include<boost/range/adaptors.hpp>
using namespace std;
namespace adaptor = boost::adaptors;
 
void Main()
{
	int R;
  
  	cin >> R;
    
    printf("%lf", 2.0 * R * 3.1415);
	
	
}
 
int main()
{
	std::cin.tie(nullptr);
	std::ios_base::sync_with_stdio(false);
	std::cout << std::fixed << std::setprecision(15);
	Main();
}