#include <bits/stdc++.h>
using namespace std;
#define rep(i, a) for (int i = 0; i < (a); i++)
#define rep2(i, a, b) for (int i = (a); i < (b); i++)
#define repr(i, a) for (int i = (a) - 1; i >= 0; i--)
#define repr2(i, a, b) for (int i = (b) - 1; i >= (a); i--)
template<class T1, class T2> bool chmin(T1 &a, T2 b) { return b < a && (a = b, true); }
template<class T1, class T2> bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
typedef long long ll;

int main() {
	int n;
	cin >> n;
	vector<int> a(n);
	rep(i, n) cin >> a[i];
	sort(a.begin(), a.end());

	vector<ll> d(5050);
	rep(i, n) rep(j, n) if (a[i] > a[j]) d[a[i] - a[j]]++;

	static ll dp[3][5050];
	dp[0][0] = 1;
	rep(i, 5050) for (int j = 0; i + j < 5050; j++) dp[1][i + j] += dp[0][j] * d[i];
	rep(i, 5050) for (int j = 0; i + j < 5050; j++) dp[2][i + j] += dp[1][j] * d[i];

	long double whole = powl(n, 3) * powl(n - 1, 3) / 8;
	repr(i, 5050 - 1) d[i] += d[i + 1];
	ll way = 0;
	for (int j = 1; j < 5050 - 1; j++) way += dp[2][j] * d[j + 1];

	long double ans = (long double)way / whole;
	printf("%.20f\n", (double)ans);

	return 0;
}