#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 7;
const int MXN = 1e6 + 7;
const double EPS = 1e-9;
const double PI = acos(-1);
long long a[MXN], cnt[MXN], n;
long long dp1[MXN], dp2[MXN], pref[MXN];
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort(a + 1, a + n + 1, greater<int>());
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      cnt[a[i] - a[j]]++;
    }
  }
  for (int i = 1; i <= 5500; i++)
    for (int j = 1; j <= 5500; j++) dp2[i + j] += cnt[i] * cnt[j];
  long double ans = 0;
  long long sum = 0;
  for (int i = 1; i <= 30000; i++) {
    ans += sum * cnt[i];
    sum += dp2[i];
  }
  long long x = (n * (n - 1ll)) / 2;
  ans /= x;
  ans /= x;
  ans /= x;
  cout << fixed << setprecision(10) << ans;
  return 0;
}
