#include <bits/stdc++.h>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;


//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()



typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
const double EPS = 1E-8;

#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
const int INF = 100000000;

struct Edge {
    int to, from;
    ll cost;
    Edge(int from, int to, ll cost): from(from), to(to), cost(cost) {}
};

class UnionFind {
public:
    vector <ll> par; 
    vector <ll> siz; 

    UnionFind(ll sz_): par(sz_), siz(sz_, 1LL) {
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }
    void init(ll sz_) {
        par.resize(sz_);
        siz.assign(sz_, 1LL);
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }

    ll root(ll x) { 
        while (par[x] != x) {
            x = par[x] = par[par[x]];
        }
        return x;
    }

    bool merge(ll x, ll y) {
        x = root(x);
        y = root(y);
        if (x == y) return false;
        if (siz[x] < siz[y]) swap(x, y);
        siz[x] += siz[y];
        par[y] = x;
        return true;
    }

    bool issame(ll x, ll y) { 
        return root(x) == root(y);
    }

    ll size(ll x) { 
        return siz[root(x)];
    }
};

typedef vector<vector<Edge>> AdjList;
AdjList graph;

ll mod_pow(ll x, ll n, ll mod){
    ll res = 1;
    bool c = false;
    while(n){
        if(n&1) res = res * x;

        if(res > mod){
            c = true;
            res %= mod;
        }
        x = x * x %mod;
        n >>= 1;
    }
    if(c) return mod;
    return res;
}

#define SIEVE_SIZE 5000000+10
bool sieve[SIEVE_SIZE];
void make_sieve(){
    for(int i=0; i<SIEVE_SIZE; ++i) sieve[i] = true;
    sieve[0] = sieve[1] = false;
    for(int i=2; i*i<SIEVE_SIZE; ++i) if(sieve[i]) for(int j=2; i*j<SIEVE_SIZE; ++j) sieve[i*j] = false;
}

bool isprime(ll n){
    if(n == 0 || n == 1) return false;
    for(ll i=2; i*i<=n; ++i) if(n%i==0) return false;
    return true;
}

template<typename T>
vector<T> gauss_jordan(const vector<vector<T>>& A, const vector<T>& b){
    int n = A.size();
    vector<vector<T>> B(n, vector<T>(n+1));

    for(int i=0; i<n; ++i){
        for(int j=0; j<n; ++j){
            B[i][j] = A[i][j];
        }
    }

    for(int i=0; i<n; ++i) B[i][n] = b[i];

    for(int i=0; i<n; ++i){
        int pivot = i;
        for(int j=i; j<n; ++j){
            if(abs(B[j][i]) > abs(B[pivot][i])) pivot = j;
        }
        swap(B[i], B[pivot]);

        if(abs(B[i][i]) < EPS) return vector<T>(); //解なし

        for(int j=i+1; j<=n; ++j) B[i][j] /= B[i][i];
        for(int j=0; j<n; ++j){
            if(i != j){
                for(int k=i+1; k<=n; ++k) B[j][k] -= B[i][j] * B[i][k];
            }
        }
    }

    vector<T> x(n);

    for(int i=0; i<n; ++i) x[i] = B[i][n];
    return x;
    
}

ll GCD(ll a, ll b){
    if(a<b) swap(a,b);
    if(b == 0) return a;
    return GCD(b, a%b);
}


typedef vector<ll> vec;
typedef vector<vec> mat;

mat mul(mat &A, mat &B) {
    mat C(A.size(), vec((int)B[0].size()));
    for(int i=0; i<A.size(); ++i){
        for(int k=0; k<B.size(); ++k){
            for(int j=0; j<B[0].size(); ++j){
                C[i][j] = (C[i][j] + A[i][k] * B[k][j] %MOD) % MOD;
            }
        }
    }
    return C;
}
mat mat_pow(mat A, ll n) {
    mat B(A.size(), vec((int)A.size()));

    for(int i=0; i<A.size(); ++i){
        B[i][i] = 1;
    }

    while(n > 0) {
        if(n & 1) B = mul(B, A);
        A = mul(A, A);
        n >>= 1;
    }
    return B;
}

bool operator<(const pii& a, const pii& b){
    if(a.first == b.first) return a.second < b.second;
    return a.first < b.first;
}

const int MAX = 510000;
long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int bit[1000010];
int sum(int i){
    int s = 0;
    while(i > 0){
        s += bit[i];
        i -= i & -i;
    }
    return s;
}
void add(int i, int x){
    while(i <= 1000010){
        bit[i] += x;
        i += i & -i;
    }
}
long long extGCD(long long a, long long b, long long &x, long long &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    long long d = extGCD(b, a%b, y, x);
    y -= a/b * x;
    return d;
}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);

    int N; cin >> N;
    vector<pii> vp;
    REP(i,N){
        int x, y; cin >> x >> y;
        vp.push_back({x, y});

    }

    sort(all(vp));

    double ans = 0;
    do{
        for(int i=0; i<N-1; i++){
            int x1 = vp[i].first, y1 = vp[i].second, x2 = vp[i+1].first, y2 = vp[i+1].second; 
            ans += sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));
        }

    }while(next_permutation(all(vp)));

    for(int i=1; i<=N; i++){
        ans /= i;
    }
    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 256