#include <iostream>
using namespace std;
#include <string>
#include <vector>
#include <algorithm>
#include <map>
typedef long long ll;
#include <cmath>
#include <iomanip>



int main(int argc, char *argv[])
{
	double n;
	cin >>n;
	
	cout<< setprecision(10)<<n*3.14159265258979323*2<<endl;
	return 0;
}

