#include <bits/stdc++.h>
using namespace std;
constexpr double PI = 3.1415926535897932;
constexpr double EPS = 1e-9;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  double r;
  cin >> r;
  double pi = acos(-1);
  double ans = r * 2 * pi;
  cout << fixed << setprecision(10);
  cout << ans << endl;
}
