#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
template <typename T, typename TT>
inline ostream &operator<<(ostream &os, const pair<T, TT> &t) {
  return os << t.first << " " << t.second;
}
template <typename T>
inline ostream &operator<<(ostream &os, const vector<T> &t) {
  for (auto i : t) os << i << " ";
  return os;
}
template <typename T>
inline ostream &operator<<(ostream &os, const set<T> &t) {
  for (auto i : t) os << i << " ";
  return os;
}
template <typename T1, typename T2>
inline ostream &operator<<(ostream &os, const map<T1, T2> &t) {
  for (auto i : t) os << i.first << " : " << i.second << '\n';
  return os;
}
template <typename T>
inline istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) is >> t;
  return is;
}
template <typename T1, typename T2>
inline istream &operator>>(istream &is, vector<pair<T1, T2>> &v) {
  for (pair<T1, T2> &t : v) is >> t.first >> t.second;
  return is;
}
const long long mod = 1e9 + 7;
void solve() {
  long long n;
  cin >> n;
  vector<long long> v(n);
  cin >> v;
  vector<vector<long long>> cnt(1e6 + 6);
  for (int i = 0; i < n; i++) cnt[v[i]].push_back(i);
  long double ans = 0;
  for (int i = 1; i <= 1e6; i++) {
    if (cnt[i].empty()) continue;
    long long cur = n * (n + 1) / 2;
    for (int j = 1; j < cnt[i].size(); j++) {
      long long len = cnt[i][j] - cnt[i][j - 1] - 1;
      cur -= len * (len + 1) / 2;
    }
    cur -= cnt[i][0] * (cnt[i][0] + 1) / 2;
    cur -= (n - cnt[i].back() - 1) * (n - cnt[i].back()) / 2;
    ans += cur;
  };
  printf("%.10Lf\n", (2 * ans - n) / (n * n));
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  ;
  long long T = 1;
  while (T--) {
    solve();
  }
  return 0;
}
