#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T, class U>
using Pa = pair<T, U>;
template <class T>
using vec = vector<T>;
template <class T>
using vvec = vector<vec<T>>;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double R;
  cin >> R;
  cout << fixed << setprecision(10) << R * 6.2831853071795 << "\n";
}
