#include <bits/stdc++.h>
using namespace std;
inline void ci(void) { return; }
template <typename First, typename... Rest>
void ci(First& first, Rest&... rest) {
  cin >> first;
  ci(rest...);
  return;
}
inline void co(void) {
  cout << "\n";
  return;
}
template <typename First, typename... Rest>
void co(First first, Rest... rest) {
  cout << first << " ";
  co(rest...);
  return;
}
int main() {
  double R;
  ci(R);
  cout << fixed << setprecision(10);
  co(R * 6.28318530717958623200);
}
