D.cpp

#include <cstdio>
#include <iostream>
#include <cmath>
#include <assert.h>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <cstring>
#include <algorithm>
#define sc scanf
#define pr printf
#define pb push_back
#define mp std::make_pair
#define fr first
#define se second

using namespace std;

typedef std::pair<int, int> pii;
typedef std::pair<double, double> pdd;

const int MN = 10010;
const long long M = -(1LL << 50);
const long long MAX_LONG = std::numeric_limits<long long>::max();
const long long MIN_LONG = std::numeric_limits<long long>::min() + (1LL << 54);
const int MAX_INT = std::numeric_limits<int>::max();
const int MIN_INT = std::numeric_limits<int>::min();

int n;
int a[MN];
int b[MN] = {0};
long long sum[MN] = {0};


int main() {
    //freopen("in.txt", "r", stdin); freopen("out.txt", "w", stdout);
    //freopen("path.in", "r", stdin); freopen("path.out", "w", stdout);
    sc("%d", &n);
    for (int i = 0; i < n; i++) {
        sc("%d", &a[i]);
    }

    sort(a, a + n);

    for(int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            b[ (a[j] - a[i]) ]++;
        }
    }

    sum[0] = b[0];

    for (int i = 1; i < MN; i++) {
        sum[i] = sum[i - 1] + b[i];
    }

    long double ans = 0.0;

    for (int i = 0; i < MN; i++) {
        if (b[i] == 0) {
            continue;
        }
        for (int j = 0; j < i; j++) {

                ans += (1.0L * (sum[i - j - 1] * b[i] * b[j]));
                //pr("%d %d: %Lf %Lf\n", i, j, (1.0 * b[i] * b[j] * sum[i - j - 1]), ans);

        }
    }

    pr("%.10Lf\n", (ans / sum[MN - 1]) / sum[MN - 1] / sum[MN - 1]);

    return 0;
}