/* Aa^~ kokoro ga pyonpyon suru n jaa^~
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*/

#include <bits/stdc++.h>
using namespace std;

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}

#ifdef PYONPOI
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)
#else
#define DEBUG
#endif
void init_io(int argc, char* argv[]){
  cin.tie(0);
  ios_base::sync_with_stdio(false);
#ifdef PYONPOI
  if(argc > 1) {
    ifstream *ifs = new ifstream();
    ifs->open(argv[1]);
    cin.rdbuf(ifs->rdbuf());
  }
#endif
}


const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

int main(int argc, char* argv[]){
  init_io(argc, argv);

  int N;
  cin >> N;
  vector<PII> xs(N);
  cin >> xs;

  double sum = 0;
  int acc = 0;
  VI ord(N);
  iota(ALL(ord), 0);
  do{
    REP(i,N-1){
      int a = ord[i];
      int b = ord[i+1];
      double dx = xs[b].FF - xs[a].FF;
      double dy = xs[b].SS - xs[a].SS;
      sum += sqrt(dx*dx+dy*dy);
    }
    acc++;
  }while(next_permutation(ALL(ord)));

  cout << fixed << setprecision(9) << sum / acc << endl;

  return 0;
}


Measured memory_bytes: 256