#include <bits/stdc++.h>
using namespace std;
namespace IO {
template <class A, class B>
ostream &operator<<(ostream &out, pair<A, B> &a) {
  out << a.first << " " << a.second;
  return out;
}
template <class A, class B>
ostream &operator<<(ostream &out, vector<pair<A, B>> &a) {
  for (pair<A, B> x : a) out << x.first << " " << x.second << '\n';
}
template <class A>
ostream &operator<<(ostream &out, vector<A> &a) {
  for (A x : a) out << x << ' ';
  return out;
}
template <class A, class B>
istream &operator>>(istream &in, pair<A, B> &a) {
  in >> a.first >> a.second;
  return in;
}
template <class A>
istream &operator>>(istream &in, vector<A> &a) {
  for (A &x : a) in >> x;
  return in;
}
}  // namespace IO
using namespace IO;
long long solve() {
  long double r;
  cin >> r;
  cout << (3.141592 * r * 2) << '\n';
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) solve();
}
