
#include <iostream>
#include <vector>
#include <math.h>
//v.erase(unique(v.begin(),v.end()),v.end());

using namespace std;
 
typedef long double ld;
typedef long long ll;
typedef  pair <ll, ll> pii;
 
const ll N = 200500;
const ll mod = 1e9 + 7;
const ld PI = 3.141592654



int main() {


	ld n;
	cin >> n;
 
	cout << (ld)2.0 * n * PI << endl;
	
	return 0;
}