#include <bits/stdc++.h>
using namespace std;
int n, a[2005];
long long sum[10005], D[5005];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  for (int i = 1; i <= n; i++)
    for (int j = i + 1; j <= n; j++) D[abs(a[i] - a[j])]++;
  sum[5000] = 0;
  double ans = 0;
  for (int i = 4999; i >= 0; i--) sum[i] = sum[i + 1] + D[i];
  for (int i = 1; i <= 5000; i++)
    for (int j = 1; i + j <= 5000; j++) ans += D[i] * D[j] * sum[i + j + 1];
  double match = n * (n - 1) / 2.0;
  printf("%.10lf\n", ans / match / match / match);
}
