#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T BM(T p, T e, T M) {
  long long int ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T MDINV(T a, T M) {
  return BM(a, M - 2, M);
}
template <class T>
inline T PW(T p, T e) {
  long long int ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p);
    p = (p * p);
  }
  return (T)ret;
}
template <class T>
string NTS(T Number) {
  stringstream ss;
  ss << Number;
  return ss.str();
}
template <class T>
T stringtonumber(const string &Text) {
  istringstream ss(Text);
  T result;
  return ss >> result ? result : 0;
}
template <class T>
bool ISLEFT(T a, T b, T c) {
  if (((a.first - b.first) * (b.second - c.second) -
       (b.first - c.first) * (a.second - b.second)) < 0.0)
    return 1;
  else
    return 0;
}
long long int ar[5005];
double ans[20000ll + 3];
void bit(long long int x, double val) {
  while (x <= 20000ll) {
    ans[x] += val;
    x += x & -x;
  }
}
double qu(long long int x) {
  double ho = 0;
  while (x > 0) {
    ho += ans[x];
    x -= (x & -x);
  }
  return ho;
}
int main() {
  int n, m;
  scanf("%d", &n);
  int a[n + 2];
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  sort(a + 1, a + n + 1);
  long long int res = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      int x = a[j] - a[i];
      ar[x]++;
      res++;
    }
  }
  for (int i = 1; i <= 5000; i++) {
    for (int j = 1; j <= 5000; j++) {
      long long int p = res * res;
      long long int x = ar[i] * ar[j];
      double co = (double)x / (double)p;
      bit(i + j, co);
    }
  }
  double sum = 0;
  for (int i = 1; i <= 5000; i++) {
    double xo = (double)ar[i] / (double)res;
    double co = qu(i - 1);
    sum += xo * co;
  }
  printf("%.10lf\n", sum);
  return 0;
}
