#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, 1, -1, -1, -1, 1, 1};
int dy[] = {1, -1, 0, 0, -1, 1, 1, -1};
template <class T>
inline T biton(T n, T pos) {
  return n | ((T)1 << pos);
}
template <class T>
inline T bitoff(T n, T pos) {
  return n & ~((T)1 << pos);
}
template <class T>
inline T ison(T n, T pos) {
  return (bool)(n & ((T)1 << pos));
}
template <class T>
inline T gcd(T a, T b) {
  while (b) {
    a %= b;
    swap(a, b);
  }
  return a;
}
template <typename T>
string NumberToString(T Number) {
  ostringstream second;
  second << Number;
  return second.str();
}
inline int nxt() {
  int aaa;
  scanf("%d", &aaa);
  return aaa;
}
inline long long int lxt() {
  long long int aaa;
  scanf("%I64d", &aaa);
  return aaa;
}
inline double dxt() {
  double aaa;
  scanf("%lf", &aaa);
  return aaa;
}
template <class T>
inline T bigmod(T p, T e, T m) {
  T ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % m;
    p = (p * p) % m;
  }
  return (T)ret;
}
int ar[100010], br[100010], great[100010];
int main() {
  int n = nxt();
  for (int i = 0; i < n; i++) ar[i] = nxt();
  int mx = 0;
  for (int i = 0; i < n; i++)
    for (int j = 0; j < n; j++) {
      if (ar[i] > ar[j] && i != j) br[ar[i] - ar[j]]++;
      mx = max(mx, ar[i] - ar[j]);
    }
  for (int i = mx; i >= 1; i--) {
    great[i] = br[i];
    great[i] += great[i + 1];
  }
  double tot = (n * (n - 1)) / 2;
  double ans = 0;
  for (int i = 1; i <= mx; i++) {
    for (int j = 1; j < mx; j++) {
      if (br[i] && br[j] && great[i + j + 1]) {
        int x = i + j;
        ans += (br[i]) / tot * (br[j]) / tot * great[x + 1] / tot;
      }
    }
  }
  cout << setprecision(20) << ans << endl;
  return 0;
}
