/*
 * Geometry: WA do sai so thap phan?
 * https://codeforces.com/contest/600/problem/D
 *
 */
/* ID: thanhvq1
LANG: C++14
PROB: namenum
*/

//#pragma GCC optimize ("O2")
#include<bits/stdc++.h>
#include<unistd.h>
using namespace std;

// defines
#define int long long
typedef long long ll;
//#define double long double
#define vec vector
#define vi vector<int>
#define vvi vector<vector<int>>
#define ii pair<int, int>
#define vii vector<ii>
#define F first
#define S second
#define MP make_pair
#define PB push_back
#define ALL(x) begin(x),end(x)
#define SZ(x) ((int)(x).size())
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define FORE(i, a, b) for(int i = (a); i <= (b); i++)
#define FORD(i, a, b) for(int i = (a); i >= (b); i--)
//#define UNI(x) (x).resize(unique((x).begin(), (x).end()) - (x).end());
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),(x))-c.begin())
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define RESET(X, y) memset(X, y, sizeof(X))
#define chkmin(a, b) a = min(a, b)
#define chkmax(a, b) a = max(a, b)
#define max3(a, b, c) max({a, b, c})
#define min3(a, b, c) min({a, b, c})

//Universal DEBUG: https://codeforces.com/blog/entry/68809
void __print(int x) {cout << x;}
void __print(long x) {cout << x;}
//void __print(long long x) {cout << x;} //invalid if #define int long long
void __print(unsigned x) {cout << x;}
void __print(unsigned long x) {cout << x;}
void __print(unsigned long long x) {cout << x;}
void __print(float x) {cout << x;}
void __print(double x) {cout << x;}
void __print(long double x) {cout << x;}
void __print(char x) {cout << '\'' << x << '\'';}
void __print(const char *x) {cout << '\"' << x << '\"';}
void __print(const string &x) {cout << '\"' << x << '\"';}
void __print(bool x) {cout << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cout << '{'; __print(x.first); cout << ','; __print(x.second); cout << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cout << '{'; for (auto &i: x) cout << (f++ ? "," : ""), __print(i); cout << "}";}
void _print() {cout << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cout << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cout << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
/* To debug, just type: debug(x, y, z...). It requires C++ 11 or above.
It can work with: Primitive data types: bool, int, long long, float, ...; pair, string;
Collection types: std::vector, std::map, std::set, ...; Expressions
*/

//permanent constants
const long long INF64 = 1e18; const int INF32 = 1e9; const int BIG = (int)(1e9 + 239);
const int MOD = 1000000007; // (int)1e9+7; //998244353; const ll MOD2 = (ll)MOD * (ll)MOD;
const double PI = 2.0*acos(0.0); //acos(-1.0L);
const double EPS = static_cast<double>(1e-10);
const int day[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int dx[4] = {0, 1, 0, -1}; const int dy[4] = {1, 0, -1, 0};

//random
mt19937 rnd(239); //(chrono::high_resolution_clock::now().time_since_epoch().count());

typedef vector<vector<int>> matrix;   //matrix Z(n, vector<LL>(n));

/*GRAPH:
	vector<vector<int>> dist; // ma tran dist giua cac dinh
    dist.assign(N, vector<int>(N, INF)); // Khoi tao ma tran dist bang INF
*/
// Sort w Lambda Function: sort(begin(v), end(v), [] (int a, int b) { return a > b; });

void FILEIO() {  freopen("in.txt", "r", stdin);  freopen("out.txt", "w", stdout); }
//void FILEIO() {  freopen("namenum.in", "r", stdin);  freopen("namenum.out", "w", stdout); }
//Tricks:
#define odd(x) ((x) & 1)
#define even(x) !((x) & 1)

int mostSignDigit(int n) { double k = log10(n); return pow(10, k-floor(k));}
int numOfDigits(int n) {return floor(log10(n)) + 1;}
bool isPowerOfTwo(int x) {return x && (!(x&(x-1)));} // first x is for the case x=0

bool allPositive(vector<int> &a) {return all_of(begin(a), end(a), [](int x){return x > 0;});} // are all of the elements positive?
bool anyPositive(vector<int> &a) {return any_of(begin(a), end(a), [](int x){return x > 0;});} // is there at least one positive element?
bool nonePositive(vector<int> &a) {return none_of(begin(a), end(a), [](int x){return x > 0;});} // are none of the elements positive?

//******************
struct Point {
	double x, y;
	Point() : x(0), y(0) {}
	Point(double x, double y) : x(x), y(y) {}
	Point(const Point & p) : x(p.x), y(p.y) {}
	bool operator< (const Point & p) const {return make_pair(x,y) < make_pair(p.x, p.y);}
	bool operator == (const Point & p) const {return make_pair(x,y) == make_pair(p.x, p.y);}
	bool operator != (const Point & p) const {return ! (*this == p); }
    Point operator = (const Point & p) { x = p.x, y = p.y;  return *this; }
    Point operator + (const Point & p) const { return Point(x+p.x, y+p.y);}
    Point operator - (const Point & p) const { return Point(x-p.x, y-p.y);}
    Point operator * (double c) const { return Point(x*c, y*c);}
    Point operator / (double c) const { return Point(x/c, y/c);}  // must be double
};
double cross(Point p, Point q) {return p.x * q.y - p.y * q.x;}
double dot(Point p, Point q) {return p.x*q.x + p.y*q.y;}
double Det(Point a, Point b, Point c) {return cross(b-a, c-a);}
double dist(Point p, Point q) {return sqrt(dot(p-q, p-q));}  // must return double
double distSquare(Point p, Point q) {return dot(p-q, p-q);}  // can return int if p, q are int-based

void solve() {
	int n; cin >> n;
	vec<Point> vp(n); FOR(i, 0, n) cin>>vp[i].x>>vp[i].y;
	vi p(n); FOR(i, 0, n) p[i]=i;
	double d = 0.0, fac = 1.0; int m = 1;
	FOR(i, 0, n) {fac *= (double)m; m++;}

//	debug(m, fac);

	do {
		FOR(i, 1, n) {
			d += dist(vp[p[i]], vp[p[i-1]]);
		}

//		debug(p);

		m++;
	} while (next_permutation(ALL(p)));
	cout << setprecision(10) << fixed << d/fac << endl;
}

signed main() {
  ios_base::sync_with_stdio(false); cin.tie();
//  FILEIO();
//  auto beginProgram = chrono::steady_clock::now();

//  int t; cin >> t;
//  FOR(i, 0, t)
  {
//	  cout << "Case #" << i+1 << ": " << solve() << endl;
	  solve();
  }

//  auto endProgram = chrono::steady_clock::now();
//  cout << chrono::duration_cast<chrono::milliseconds>(endProgram - beginProgram).count() << " ms" << endl;

  return 0;
}


Measured memory_bytes: 256