#include <bits/stdc++.h>
int nextInt() {
  int x;
  scanf("%d", &x);
  return x;
}
double nextDouble() {
  double x;
  scanf("%lf", &x);
  return x;
}
long long nextLong() {
  long long x;
  scanf("%I64d", &x);
  return x;
}
char nextChar() {
  char x;
  scanf("%c", &x);
  return x;
}
void newLine() { printf("\n"); }
long long powM(long long a, long long b, long long MOD) {
  long long x = 1, y = a;
  while (b > 0) {
    if (b % 2 == 1) {
      x = (x * y);
      if (x > MOD) x %= MOD;
    }
    y = (y * y);
    if (y > MOD) y %= MOD;
    b /= 2;
  }
  return x;
}
int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
int countBitSet(long long n) {
  int ans = 0;
  while (n != 0) {
    n -= (n & -n);
    ++ans;
  }
  return ans;
}
long long mod = long long(1e9 + 7);
long long mod2 = long long(1e9 + 9);
const int N = 5111;
const int M = 2111;
const int LOG = 21;
const long long INF = -1e18;
const long double EPS = 1e-9;
const long double PI = 2 * acos(0);
using namespace std;
int a[M];
int occ[N];
int b[N];
int main() {
  cout << fixed << setprecision(20);
  long long n = nextLong();
  for (int i = 0; i < n; i++) {
    a[i] = nextInt();
  }
  if (n < 3) {
    cout << 0 << endl;
    return 0;
  }
  sort(a, a + n);
  for (int i = 0; i < n; i++) {
    for (int j = i + 1; j < n; j++) {
      occ[a[j] - a[i]]++;
    }
  }
  b[N - 1] = 0;
  for (int i = N - 2; i >= 0; i--) {
    b[i] = occ[i] + b[i + 1];
  }
  double numerator = 0;
  for (int i = 1; i < N; i++) {
    for (int j = 1; j < N; j++) {
      numerator += 1ll * occ[i] * occ[j] * 1ll * b[j + i + 1];
    }
  }
  long long denominator = n * (n - 1) / 2;
  denominator *= denominator * denominator;
  cout << numerator / denominator << endl;
  return 0;
}
