#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(i64 (i) = (s);(i) < (e);(i)++)
#define all(x) x.begin(),x.end()

template<class T>
static inline std::vector<T> ndvec(size_t&& n, T val) noexcept {
  return std::vector<T>(n, std::forward<T>(val));
}

template<class... Tail>
static inline auto ndvec(size_t&& n, Tail&&... tail) noexcept {
  return std::vector<decltype(ndvec(std::forward<Tail>(tail)...))>(n, ndvec(std::forward<Tail>(tail)...));
}

template<class T, class Cond>
struct chain {
  Cond cond; chain(Cond cond) : cond(cond) {}
  bool operator()(T& a, const T& b) const {
    if(cond(a, b)) { a = b; return true; }
    return false;
  }
};
template<class T, class Cond>
chain<T, Cond> make_chain(Cond cond) { return chain<T, Cond>(cond); }

int main() {
  double R;
  cin >> R;
  std::cout << fixed << setprecision(10) << std::endl;
  std::cout << R * 2 * std::acos(-1) << std::endl;
}
