#include <bits/stdc++.h>
using namespace ::std;
template <class T>
istream& operator>>(istream& I, pair<T, T>& p) {
  return I >> p.first >> p.second;
}
template <class T>
ostream& operator<<(ostream& O, pair<T, T>& p) {
  return O << p.first << ' ' << p.second;
}
template <class T>
istream& operator>>(istream& I, vector<T>& v) {
  for (T& e : v) I >> e;
  return I;
}
template <class T>
ostream& operator<<(ostream& O, const vector<T>& v) {
  for (const T& e : v) O << e << ' ';
  return O;
}
void solve() {
  double r;
  cin >> r;
  cout << M_PI * r * 2;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) solve();
  return 0;
}
