#include <bits/stdc++.h>
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;
int main() {
  double R;
  cin >> R;
  cout << std::fixed << std::setprecision(4) << 2 * R * 3.14159263 << endl;
  return 0;
}
