#include <bits/stdc++.h>
using namespace std;
template <typename T0, typename T1>
std::ostream& operator<<(std::ostream& os, const map<T0, T1>& v) {
  for (typename map<T0, T1>::const_iterator p = v.begin(); p != v.end(); p++) {
    os << p->first << ": " << p->second << " ";
  }
  return os;
}
template <typename T0, typename T1>
std::ostream& operator<<(std::ostream& os, const pair<T0, T1>& v) {
  os << v.first << ": " << v.second << " ";
  return os;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, const vector<T>& v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << " ";
  }
  return os;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, const set<T>& v) {
  vector<T> tmp(v.begin(), v.end());
  os << tmp;
  return os;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, const deque<T>& v) {
  vector<T> tmp(v.begin(), v.end());
  os << tmp;
  return os;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, const vector<vector<T>>& v) {
  for (int i = 0; i < (int)v.size(); i++) {
    os << v[i] << endl;
  }
  return os;
}
template <typename T>
void maxUpdate(T& a, T b) {
  a = max(a, b);
}
template <typename T>
void minUpdate(T& a, T b) {
  a = min(a, b);
}
string YN(bool v) { return v ? "Yes" : "No"; }
void uniq(vector<long long>& es) {
  sort((es).begin(), (es).end());
  es.erase(unique((es).begin(), (es).end()), es.end());
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long N, M, W, H, K;
  string s;
  while (cin >> N) {
    double ans = M_PI * 2 * N;
    cout << ans << endl;
  }
  return 0;
}
