#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define INF 999999999
#define M_PI 3.14159265358979323846
const double eps = 1e-9;

#ifdef __LOCAL
    #define DBG(X) cout << #X << " = " << (X) << endl;
    #define SAY(X) cout << (X) << endl;
#else
    #define DBG(X)
    #define SAY(X)
#endif

#ifdef __LOCAL
    #include <filesystem>
    namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[]={1, 0, -1, 0, 0};
int dy[]={0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
 
std::string pad(int num){ char buffer[4]; std::snprintf(buffer, sizeof(buffer), "%03d", num); return buffer;}
inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }


struct point { double x, y; };
 
point  operator+(point a,  point b) { return {a.x+b.x, a.y+b.y}; }
point  operator-(point a,  point b) { return {a.x-b.x, a.y-b.y}; }
point  operator*(double t, point b) { return {t*b.x, t*b.y}; }
point  operator/(point a, double t) { return {a.x/t, a.y/t}; }
 
double operator*(point a,  point b) { return a.x*b.x + a.y*b.y; } // dot product
double operator%(point a,  point b) { return a.x*b.y - a.y*b.x; } // cross product
 
bool operator<(point a, point b) { // lexicographical compare
	if (abs(a.x-b.x) > eps) return a.x < b.x;
	return a.y + eps < b.y;
}
 
ostream &operator<<(ostream &out, point a) { // for demplace_backugging
	return out << "(" << a.x << "," << a.y << ")";
}
 
double abs(point a) { return hypot(a.x,a.y); }
point perp(point a) { return {-a.y,a.x}; } // rotate 90 degrees counterclockwise
point normalize(point a) { return a/abs(a); }
double dist(point a, point b){ return abs(a-b);}

int fact_d[10];
int fact(int const& n){
    if(fact_d[n]) return fact_d[n];
    else if(n==1) return 1;
    else return fact_d[n] = n*fact(n-1);
}

int main()
{
    #ifdef __LOCAL
        fs::path p = __FILE__;
        fs::path input,output;
        input = output = p.parent_path();
        input += string("/input/") + string(p.stem()) + string(".txt");
        output += string("/output/") + string(p.stem()) + string(".txt");
        freopen(input.c_str(), "r", stdin);
        freopen(output.c_str(), "w", stdout);
    #endif

    int N;
    cin >> N;
    vector<point> A(N);
    FOR(i,0,N){
        cin >> A[i].x;
        cin >> A[i].y;
    }

    double sum;
    sum = 0;
    FOR(i,0,N) FOR(j,0,N) sum += dist(A[i],A[j]);
    sum /= N;
    cout << fixed << setprecision(20) << sum << endl;

    return 0;
}


Measured memory_bytes: 256