#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void out(T t) {
  cout << t << "\n";
}
template <class T, class... Ts>
inline void out(T t, Ts... ts) {
  cout << t << " ";
  out(ts...);
}
template <class T>
inline bool CHMIN(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool CHMAX(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
constexpr long long INF = 1e9;
signed main() {
  long long r;
  cin >> r;
  printf("%.20lf\n", 2 * r * acos(-1));
}
