#include <bits/stdc++.h>
using namespace std;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <class BidirectionalIterator>
bool next_partial_permutation(BidirectionalIterator first,
                              BidirectionalIterator middle,
                              BidirectionalIterator last) {
  reverse(middle, last);
  return next_permutation(first, last);
}
void Main() {
  double R;
  double length;
  cin >> R;
  length = 2 * M_PI * R;
  cout << length << "\n";
  return;
}
int main() {
  std::cin.tie(0);
  std::ios_base::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);
  Main();
}
