#include <bits/stdc++.h>

#define PI acos(-1)
#define all(a) a.begin(), a.end()
#define full(a, n) a, a + n
#define ma(type, size) (type *)malloc(sizeof(type) * size)
#define F first
#define S second
#define PB push_back
#define MP make_pair
#define MT make_tuple
#define f0(i, n) for (i = 0; i < (n); i++)
#define f1(i, n) for (i = 1; i < (n); i++)
#define f1n(i, n) for (i = 1; i <= (n); i++)
#define fab(i, a, b) for (i = (a); i <= (b); i++)
#define fr(i, b, a) for (i = (b); i >= (a); i--)

using namespace std;

/*
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,
	tree_order_statistics_node_update> indexed_set;
//*/
typedef long long int ll;
typedef long int lo;

typedef vector<ll> vll;
typedef pair<ll, ll> pll;
typedef tuple<ll, ll, ll> tll;
/*
set<int> operator|(set<int> s1, set<int> s2) {
    set<int> ret;
    set_union(s1.begin(), s1.end(), s2.begin(), s2.end(), ::inserter(ret,ret.begin()));
    return ret;
}
set<int> operator&(set<int> s1, set<int> s2) {
    set<int> ret;
    set_intersection(s1.begin(), s1.end(), s2.begin(), s2.end(), ::inserter(ret,ret.begin()));
    return ret;
}//*/


//* utility function
inline bool compare(ll i, ll j)
{
    return i < j;
}
//*/

//* function prototypes
template< typename T>
T gcd(T a, T b);
template< typename T>
T modpow(T number, T power, T mod_value);
//*/

//int row, column;
//int dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
//int dy[8] = {0, -1, -1, -1, 0, 1, 1, 1};
//int dx[4] = {1,  0, -1, 0};
//int dy[4] = {0, -1,  0, 1};
//ll p2[63] = {1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648, 4294967296, 8589934592, 17179869184, 34359738368, 68719476736, 137438953472, 274877906944, 549755813888, 1099511627776, 2199023255552, 4398046511104, 8796093022208, 17592186044416, 35184372088832, 70368744177664, 140737488355328, 281474976710656, 562949953421312, 1125899906842624, 2251799813685248, 4503599627370496, 9007199254740992, 18014398509481984, 36028797018963968, 72057594037927936, 144115188075855872, 288230376151711744, 576460752303423488, 1152921504606846976, 2305843009213693952, 4611686018427387904};

#ifdef DEBUG
#endif // DEBUG
inline double dis(pair<double, double>p1, pair<double, double>p2)
{
    return sqrt((p1.F-p2.F)*(p1.F-p2.F) + (p1.S-p2.S)*(p1.S-p2.S));
}
int main()
{
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);

    ll fact[9];
    fact[0] = 1;

    for (ll i = 1; i <9; i++)fact[i] = fact[i-1]*i;

    ll number_of_test_case = 1, test_case;
    //cin >> number_of_test_case;
    for (test_case = 1; test_case <= number_of_test_case; test_case++)
    {
        ll i, j, n;

        cin >> n;

        int index[n];
        f0(i, n)index[i] = i;
        pair<double, double> coor[n];
        f0(i, n)cin >> coor[i].first >> coor[i].second;

        ll mul = fact[n-1]*2;
        double total_distance = 0;

        for (i = 0; i < n; i++)
        {
            for (j = i+1; j < n; j++)
            {
                total_distance += dis(coor[i], coor[j])*mul;
            }
        }
        cout << fixed << setprecision(10) << total_distance/fact[n] << endl;

    	//cout << "Test #" << test_case << ": ";
    	//cout << "Case " << test_case << ": ";
    }
    return 0;
}


template< typename T>
T gcd(T a, T b)
{
    if (b == 0)
        return a;
    return gcd(b, a%b);
}
template< typename T>
T modpow(T number, T power, T mod_value)
{
    if (power == 0)
        return 1%mod_value;
    ll u = modpow(number,power/2,mod_value);
    u = (u*u)%mod_value;
    if (power%2 == 1)
        u = (u*number)%mod_value;
    return u;
}


Measured memory_bytes: 256