#include <iostream>
#include <algorithm>
#include <vector>
#include <string.h>
#include <cstdlib>
#include <map>
#include <fstream>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <random>
#include <set>
#include <queue>
#include <bitset>
#include <stack>


using namespace std;

long long dp[3][100100] = {};
const long long INF = 1LL << 60;
#define MOD 1000000007
#define Int int64_t
#define PI 3.14159265358979
#define dump(a) cout << a << endl;

template<class T>
inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

/**
* @fn
* 最大値を取ります.Vector<int>を渡すこと.
*/
long long int imax(vector<int> a) {
    int MAX = -10000000;
    for (int i = 0; i < a.size(); i++) {
        if (MAX < a.at(i)) MAX = a.at(i);
    }
    return MAX;
}

/**
* @fn
* 最小値を取ります.Vector<int>を渡すこと.
*/
long long int imin(vector<int> a) {
    int MIN = 1000000000;
    for (int i = 0; i < a.size(); i++) {
        if (MIN > a.at(i)) MIN = a.at(i);
    }
    return MIN;
}

/**
* @fn
* 自然数について、桁数を求める
*/
Int getDigit(Int num) {
    return (Int) (log10(num)) + 1;
}


int get2Digit(Int num) {
    int cnt = 0;
    while (num > 0) {
        if (num % 2 != 0) {
            return cnt;
        }
        cnt++;
        num /= 2;
    }
    return cnt;
}

long long int absmin(vector<int> a, long long int b) {
    int mini = 100000000;
    int d = 0;
    for (int i = 0; i < a.size(); i++) {
        if (mini > abs(a.at(i) - b)) {
            mini = abs(a.at(i) - b);
            d = i;
        }
    }
    return d;
}


long long int absmax(vector<int> a, long long int b) {
    int max = -100000000;
    int d = 0;
    for (int i = 0; i < a.size(); i++) {
        if (max < abs(a.at(i) - b)) {
            max = abs(a.at(i) - b);
            d = i;
        }
    }
    return d;
}

int gcd(int a, int b) {
    if (a < b) swap(a, b);
    if (b < 1) return -1;
    if (a % b == 0) return b;
    return gcd(b, a % b);
}

void haerobatya(int start, int end, char level) {
    if (start > end) swap(start, end);
    for (int i = start; i <= end; i++) {
        if (i < 100) {
            cout << "https://atcoder.jp/contests/abc0" << i << "/tasks/abc0" << i << "_" << level << endl;
        } else {
            cout << "https://atcoder.jp/contests/abc" << i << "/tasks/abc" << i << "_" << level << endl;
        }
    }
}

long long int cntmod(long long int warareru, long long int mod) {
    long long int i = 0;
    for (i = 0; true; i++) {
        if (warareru % mod == 0 && warareru >= mod) {
            warareru /= mod;
        } else {
            return i;
        }
    }
}

void ansYes(bool a) {
    if (a) cout << "Yes" << endl;
    else cout << "No" << endl;
}

void ansYES(bool a) {
    if (a) cout << "YES" << endl;
    else cout << "NO" << endl;
}


Int euclidean_gcd(Int a, Int b) {
    while (true) {
        if (a < b) swap(a, b);
        if (!b) break;
    }
    return a;
}

//UnionFind群
class UnionFind {
protected:
    vector<int> par;
    vector<int> data;
    int cnt;
public:
    UnionFind() : par(1), data(1), cnt(1) {};

    UnionFind(int N);

    UnionFind(const vector<int> &par, const vector<int> &data, int cnt);

    int root(int x);

    int marge(int x, int y);

    bool same(int x, int y);

    int add();

    int add(int x);

    inline int getSize(int x) { return data.at(root(x)); };

    int allroot();

    inline bool isroot(int x) { return (par.at(x) == -1); };
};


UnionFind::UnionFind(int N) : par(N), data(N), cnt(N) {
    for (int i = 0; i < N; i++) {
        par.at(i) = -1;
        data.at(i) = 1;
    }
}


int UnionFind::root(int x) {
    if (par.at(x) == -1) return x;
    return par.at(x) = root(par.at(x)); // 経路圧縮
}


int UnionFind::marge(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    if (rx == ry) return rx;
    else { // ノード数の少ない方に接続
        if (data.at(rx) < data.at(ry)) swap(rx, ry);
        par.at(ry) = rx;
        data.at(rx) += data.at(ry);
        return rx;
    }
}

bool UnionFind::same(int x, int y) {
    return root(x) == root(y);
}

int UnionFind::add() {
    par.push_back(cnt);
    cnt++;
    return cnt - 1;
}

int UnionFind::add(int x) {
    par.push_back(cnt);
    cnt++;
    return marge(x, cnt);
}

int UnionFind::allroot() {
    int cnt = 0;
    for (const auto it : par) {
        if (it == -1) cnt++;
    }
    return cnt;
}


//約数の数を数える
int yakusu(int n) {
    int cnt = 0;
    for (int i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            cnt++;
            if (i * i != n) cnt++;
        }
    }
    return cnt;
}

int vecsum(vector<int> a) {
    int res = 0;
    for (auto z : a){
        res += z;
    }
    return res;
}

vector<int> yakusurekkyo(int n) {
    vector<int> a;
    for (int i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            a.push_back(i);
            if (i * i != n) a.push_back(n / i);
        }
    }
    return a;
}


stack<int> sc;
double dfs (double x,int z) {
    for (int i = z + 1;  i <= 8;i++){
        return dfs(x,z+1);
    }
    return 0;
}

int main() {
    int n;
    cin >> n;
    long double ans = 0;
    vector<long double> x(n);
    vector<long double> y(n);
    for (int i = 0; i < n; i++){
        cin >> x.at(i) >> y.at(i);
    }
    for (int i = 0; i < n;i++){
        for (int j = 0; j < n;j++) {
            if (i == j) continue;
            ans += sqrt(pow(x.at(i) - x.at(j),2) + pow(y.at(i) - y.at(j),2));
        }
    }
    ans =  ans / n;
    cout << setprecision(9) << ans << endl;
}







Measured memory_bytes: 256