#define _CRT_DISABLE_PERFCRIT_LOCKS
#pragma comment(linker, "/STACK:16777216")

#include <cctype>
#include <vector>
#include <map>
#include <set>
#include <complex>
#include <ctime>
#include <iostream>
#include <fstream>
#include <time.h>
#include <cmath>
#include <stack>
#include <stdio.h>
#include <list>
#include <algorithm>
#include <unordered_map>
#include <queue>
#include <unordered_set>
#include <random>


const long double PI(acos(-1.0));

#define pb push_back
#define mp(a,b) make_pair(a,b)
#define all(x) x.begin(), x.end()
#define sqr(x) ((x)*(x))
#define F first
#define S second
#define eps 1e-14
#define inf (int)(1e9+7)
#define infll (ll)(1e18+3)
#define sz(x) ((int)x.size())
#define bits(x) __builtin_popcount(x)
#define bitsl(x) __builtin_popcountll(x)


using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
typedef vector < ll > vll;
typedef vector<int> vi;
typedef pair < ll, ll > pll;
typedef pair < int, int > pii;
typedef vector<vi> vii;
typedef int huint;

const int N = 5050;
ll sums[N];
ll prs[N<<1];
ll smtr[N<<1];
ll allprs = 0;

int main() {
#ifndef LOCAL_COMP
    //    freopen("evacuate.in", "r", stdin);
    //    freopen("evacuate.out", "w", stdout);
#else
    freopen("/Users/nikita/Desktop/acm/acm/input.txt", "r", stdin);
    //freopen("/Users/nikita/Desktop/acm/acm/output.txt", "w", stdout);
#endif
    cout.precision(10);


    int n; cin >> n;
    allprs = n*(n-1)/2;
    vi a(n);
    for (int i = 0; i<n; ++i)
        cin >> a[i];
    for (int i = 0; i<n; ++i)
        for (int j = i+1; j<n; ++j)
            sums[abs(a[i]-a[j])]++;

    for (int i = 0; i<N-20; ++i) {
        for (int j = i; j<N-20; ++j) {
            prs[i+j] += sums[i] * sums[j];
        }
    }
    for (int i = (N<<1)-20; i>=0; --i)
        smtr[i] = sums[i] + smtr[i+1];


    ld ans = 0;
    for (int i = 1; i<N-20; ++i) {
        if (!sums[i] || !smtr[i+1] || !prs[i]) continue;
        ld verchspr = (prs[i] + .0) / (allprs + .0);
        verchspr /= (allprs + .0);
        verchspr *= (smtr[i+1] + .0);
        verchspr /= (allprs + .0);
        ans += verchspr;
    }
    cout << fixed << ans / allprs;



#ifdef LOCAL_COMP
    cerr << "\n == TIME : " << clock() / ld(CLOCKS_PER_SEC) << " == " << endl;
#endif
}