#include <iostream>
#include <cmath>
#include <cstdlib>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <iomanip>

#define pb push_back
#define mp make_pair
#define forn(i,n) for (int i = 0; i < n; i++)

using namespace std;

typedef pair <int, int> pii;
typedef long double ld;
typedef long long ll;

const ld EPS = 1e-9;
const int INF = (int) 1e9;
const int N = (int) 1e6 + 13;
const ll M = (int) 1e9 + 7;

int sum[N], raz[N];
int a[N];
ld prob[N];

int main() {
    int n;
    cin >> n;
    for (int i = 0; i < n; i++)
        cin >> a[i];
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
            if (a[i] - a[j] > 0)
                raz[a[i] - a[j]]++;
    for (int i = 0; i < 5010; i++)
        for (int j = 0; j < 5010; j++)
            if (raz[i] && raz[j])
                sum[i + j]++, prob[i + j] = 1.0 * raz[i] * 1.0 / (n * (n - 1) / 2) * (raz[j] * 1.0 / (n * (n - 1) / 2));

    ld ans = 0, cnt = 8.0 / (n * n * n * (n - 1) * (n - 1) * (n - 1));
    for (int i = 0; i < 5010; i++)
        for (int j = 0; j < 10010; j++)
            if (i - j < 0 && raz[i] && sum[j])
                ans += prob[j] * raz[i]*1.0 / (n * (n - 1) / 2);
    cout.precision(10);
    cout << fixed << ans;


	return 0;
}