#include <bits/stdc++.h>
using ll = long long;
template <class T>
using vt = std::vector<T>;
using vvi = std::vector<vt<int>>;
int main() {
  double R;
  std::cin >> R;
  std::cout << std::fixed << std::setprecision(10) << '\n';
  double pi = 3.1415926535;
  std::cout << 2.0 * R * pi << '\n';
  return 0;
}
