#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline void dump(T& v) {
  for (auto i = v.begin(); i != v.end(); i++) {
    cout << *i << ((i == --v.end()) ? '\n' : ' ');
  }
}
inline string getline() {
  string s;
  getline(cin, s);
  return s;
}
inline void yn(const bool b) {
  b ? cout << ("yes") << '\n' : cout << ("no") << '\n';
}
inline void Yn(const bool b) {
  b ? cout << ("Yes") << '\n' : cout << ("No") << '\n';
}
inline void YN(const bool b) {
  b ? cout << ("YES") << '\n' : cout << ("NO") << '\n';
}
struct io {
  io() {
    ios::sync_with_stdio(false);
    cin.tie(0);
  }
};
const int INF = INT_MAX;
const long long int LLINF = 1LL << 60;
const long long int MOD = 1000000007;
const double EPS = 1e-9;
int main() {
  double r;
  cin >> r;
  cout << r * 2 * acos(-1) << endl;
}
