#undef _GLIBCXX_DEBUG

#include "bits/stdc++.h"
#include "ext/pb_ds/assoc_container.hpp"
#include "ext/pb_ds/tree_policy.hpp"
#include "ext/rope"

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;

void trace(int x)                         { cerr << x; }
void trace(long x)                        { cerr << x; }
void trace(long long x)                   { cerr << x; }
void trace(unsigned x)                    { cerr << x; }
void trace(unsigned long x)               { cerr << x; }
void trace(unsigned long long x)          { cerr << x; }
void trace(float x)                       { cerr << x; }
void trace(double x)                      { cerr << x; }
void trace(long double x)                 { cerr << x; }
void trace(char x)                        { cerr << '\'' << x << '\''; }
void trace(const char *x)                 { cerr << '\"' << x << '\"'; }
void trace(const string &x)               { cerr << '\"' << x << '\"'; }
void trace(bool x)                        { cerr << (x ? "true" : "false"); }

template <typename A, typename B>                         void trace(const pair <A, B> &x)         { cerr << '{'; trace(x.first); cerr << ','; trace(x.second); cerr << '}'; }
template <typename A, typename B, typename C>             void trace(const tuple <A, B, C> &x)     { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << '}'; }
template <typename A, typename B, typename C, typename D> void trace(const tuple <A, B, C, D> &x)  { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << ','; trace(get <3> (x)); cerr << '}'; }
template <typename T>                                     void trace(const T &x)                   { int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), trace(i); cerr << "}"; }
template <size_t N>                                       void trace(bitset <N> v)                 { cerr << '{'; for (size_t i = 0; i < N; i++) cerr << static_cast <char> ('0' + v[i]); cerr << '}'; }
void _print()                                                                                      { cerr << "]\n"; }
template <typename T, typename... V>                      void _print(T t, V... v)                 { trace(t); if (sizeof...(v)) cerr << ", "; _print(v...); }

#ifndef ONLINE_JUDGE
      #define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
      #define debug(x...)
#endif // ONLINE_JUDGE

template <typename T> using ordered_set                = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T1, typename T2> using ordered_map  = tree <T1, T2, less <T1>, rb_tree_tag, tree_order_statistics_node_update>;

template <class T> T binaryExpo(T a, T p)              { if (p == 0) { return 1LL; } if (p == 1) { return a; } if (p & 1) { return a * binaryExpo(a, p - 1); } T ret = binaryExpo(a, p / 2); return ret * ret; }
template <class T> T bigMod(T a, T p, T m)             { if (p == 0) { return 1LL % m; } if (p == 1) { return a % m; } if (p & 1) { return (a % m * bigMod(a, p - 1, m)) % m; } T ret = bigMod(a, p / 2, m) % m; return (ret * ret) % m; }
template <class T> T modInv(T a, T m)                  { return bigMod(a, m - 2, m); }

template <class T> bool MIN(T &a, T b)                 { return a > b ? (a = b, true) : false; }
template <class T> bool MAX(T &a, T b)                 { return a < b ? (a = b, true) : false; }

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

inline long long ON(long long mask, int pos)           { return mask |= (1LL << pos); }
inline long long OFF(long long mask, int pos)          { return mask &= ~(1LL << pos); }
inline long long TOGGLE(long long mask, int pos)       { return mask ^= (1LL << pos); }
inline bool CHECK(long long mask, int pos)             { return (bool)(mask >> pos & 1LL); }

inline int ONE(long long mask)                         { return __builtin_popcountll(mask); }
inline int LZERO(long long mask)                       { return 31 - __builtin_clzll(mask); }
inline int TZERO(long long mask)                       { return __builtin_ctzll(mask); }

#define ll              long long int
#define ull             unsigned long long int
#define ld              long double
#define endl            '\n'
#define pii             pair <int, int>
#define pll             pair <ll, ll>
#define mk              make_pair
#define ff              first
#define ss              second
#define all(a)          (a).begin(), (a).end()
#define rall(a)         (a).rbegin(), (a).rend()
#define eb              emplace_back
#define pb              push_back
#define pf              push_front
#define sz(a)           (int)(a.size())
#define ook             order_of_key             // The number of items in a set that are strictly smaller than k
#define fbo             find_by_order            // It returns an iterator to the i'th largest element
#define ATCODER         if (fopen("in.txt", "r")) { freopen("in.txt", "r", stdin); }

#define int             long long int

const int maxn = 2e5 + 5;
const int logn = 19;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;

signed main()
{
      ios_base::sync_with_stdio(false);
      cin.tie(nullptr); cout.tie(nullptr);
      cout.precision(12);

      bool FILEIO = 0;
      if (FILEIO and fopen("in.txt", "r")) {
            freopen("in.txt", "r", stdin);
      }

      int n;
      cin >> n;
      vector <double> X(n), Y(n);
      vector <int> permuatation(n);
      for (int i = 0; i < n; i++) {
            cin >> X[i] >> Y[i];
            permuatation[i] = i;
      }
      auto dist = [&](int i, int j) {
            ld x = X[i] - X[j];
            ld y = Y[i] - Y[j];
            return sqrt(x * x + y * y);
      };
      ld sum = 0;
      ld cnt = 0;
      do {
            ld nsum = 0;
            for (int i = 1; i < n; i++) {
                  nsum += dist(permuatation[i - 1], permuatation[i]);
            }
            sum += nsum;
            cnt++;
      } while (next_permutation(all(permuatation)));
      ld mean = sum / cnt;
      cout << fixed << setprecision(10) << mean << endl;

      #ifndef ONLINE_JUDGE
            cerr << endl << "Time : " << 1.0 * clock() / CLOCKS_PER_SEC << " s." << endl;
      #endif // ONLINE_JUDGE
}


Measured memory_bytes: 3828