#include <bits/stdc++.h>
const double EPS = 1e-10;
using namespace std;
struct mint {
  long long x;
  mint(long long x = 0) : x((x % 1000000007 + 1000000007) % 1000000007) {}
  mint operator-() const { return mint(-x); }
  mint& operator+=(const mint a) {
    if ((x += a.x) >= 1000000007) x -= 1000000007;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += 1000000007 - a.x) >= 1000000007) x -= 1000000007;
    return *this;
  }
  mint& operator*=(const mint a) {
    (x *= a.x) %= 1000000007;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(long long t) const {
    if (!t) return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1) a *= *this;
    return a;
  }
  mint inv() const { return pow(1000000007 - 2); }
  mint& operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x; }
ostream& operator<<(ostream& os, const mint& a) { return os << a.x; }
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& V) {
  long long N = V.size();
  for (long long i = 0; i < N - 1; i++) {
    os << V[i] << ' ';
  }
  os << V[N - 1];
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<vector<T>>& V) {
  long long N = V.size();
  for (long long i = 0; i < N - 1; i++) cout << V[i] << "\n";
  cout << V[N - 1];
  return os;
}
template <typename T, typename S>
ostream& operator<<(ostream& os, pair<T, S> const& P) {
  os << P.first << ' ' << P.second;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, set<T>& S) {
  auto it = S.begin();
  while (it != S.end()) {
    os << *it;
    if (next(it, 1) != S.end()) os << ' ';
    it++;
  };
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, deque<T>& q) {
  for (auto it = q.begin(); it < q.end(); it++) {
    os << *it;
    if (it + 1 != q.end()) os << " ";
  }
  return os;
}
signed main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(12);
  double r;
  cin >> r;
  cout << (2 * 3.141592653589793238462 * r) << "\n";
  return 0;
}
