#include <iostream>
#include <math.h>
#include <vector>
#include <algorithm>


typedef long long ll;

using namespace std;

#define PI 3.1415 
  
double circumference(double r) 
{ 
    double cir = 2*PI*r; 
    return cir; 
} 

int main() 
{ 

	double r;
	cin >> r;

	cout << circumference(r);

	return 0; 
} 
