#include <bits/stdc++.h>
using namespace std;
class ACirclePond {
 public:
  void solve(std::istream &in, std::ostream &out) {
    int r;
    in >> r;
    out << setprecision(12) << fixed << 2.0 * r * M_PI << '\n';
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ACirclePond solver;
  std::istream &in(std::cin);
  std::ostream &out(std::cout);
  solver.solve(in, out);
  return 0;
}
