#include <iostream>
#include <vector> 
#include <set>
#include <map>
#include <math.h>

using namespace std;

#define loop(i,a,b) for(int i = a; i<b; i++)
#define square(a) (a)*(a)

typedef long long ll;
typedef string str;


int main ()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	int r;
	cin >> r;
	
	const long double pi = 3.141592653;
	cout << (long double)r * 2 * pi;
	
    return 0;
	
}
