#include <bits/stdc++.h>
#define mp make_pair
#define pb push_back
#define f first
#define s second
using namespace std;
typedef long long ll;
const int INF = 1e9;
const ll INFl = 1e18;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef set<int> si;
typedef map<int,int> mii;
typedef unordered_map<int,int> umii;
typedef queue<int> qi;

int main() {
	ios::sync_with_stdio(false); cin.tie(0);
	long double x;
	cin >> x;
	long double y = 2.00000 *  3.14159265359 * x;
	cout << fixed << setprecision(6) << y << '\n';
	
	return 0;
}
