#include <bits/stdc++.h>
#define BIT(n) (1LL << (n))
#define BITF(n, i) (((n) >> i) & 1)
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPB(i, n) for (int i = 0; i < BIT(n); i++)
#define REPS(i, x) for (int i = 1; i <= x; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define REPZ(i, x) for (int i = 0; i <= x; i++)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define FORA(i, n) for (auto &&i : n)
#define FORS(i, m, n) for (int i = m; i <= n; i++)
using namespace std;
#define DUMPOUT cerr
// vector
template < typename T > istream &operator>>(istream &is, vector< T > &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
// pair
template < typename T, typename U >
ostream &operator<<(ostream &os, pair< T, U > &pair_var) {
  os << "(" << pair_var.first << ", " << pair_var.second << ")";
  return os;
}
// vector
template < typename T >
ostream &operator<<(ostream &os, const vector< T > &vec) {
  os << "{";
  REP(i, (int)vec.size())
  os << vec[i] << (i + 1 == (int)vec.size() ? "" : ", ");
  os << "}";
  return os;
}
// map
template < typename T, typename U >
ostream &operator<<(ostream &os, map< T, U > &map_var) {
  os << "{";
  FORA(itr, map_var) {
    os << *itr;
    itr++;
    if (itr != map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
// set
template < typename T > ostream &operator<<(ostream &os, set< T > &set_var) {
  os << "{";
  FORA(itr, set_var) {
    os << *itr;
    itr++;
    if (itr != set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
void dump_func() { DUMPOUT << endl; }
template < class Head, class... Tail >
void dump_func(Head &&head, Tail &&... tail) {
  DUMPOUT << head;
  if (sizeof...(Tail) > 0)
    DUMPOUT << ", ";
  dump_func(std::move(tail)...);
}
#ifdef DEBUG_
#define DEB
#define DUMP(...)                                                              \
  DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                              \
          << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl  \
          << "    ",                                                           \
      dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define DUMP(...)
#endif

#define ALL(v) v.begin(), v.end()
#define fst first
#define snd second
#define mp make_pair
#define pb push_back
#define epb emplace_back
#define int long long
#define pint pair< int, int >
#define ld long double
using namespace std;
template < class T > bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template < class T > bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template < class T > using vec = std::vector< T >;
template < class T > void print(const T &x) { cout << x << "\n"; }
const int MOD = 1000000007, INF = 1061109567, INF2 = INF * INF;
const double EPS = 1e-10, PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
#define MAXN
//----//
signed main() {
  cin.tie(0), ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  int N;
  cin >> N;
  vec< int > x(N), y(N);
  REP(i, N) { cin >> x[i] >> y[i]; }
  double ans = 0;
  REP(i, N) {
    FOR(j, i + 1, N) {
      int dx = x[j] - x[i];
      int dy = y[j] - y[i];
      ans += sqrt(dx * dx + dy * dy);
    }
  }
  ans *= 2.0 / N;
  print(ans);
}

Measured memory_bytes: 256