#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T>
inline bool chmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}
template <class T>
using MaxHeap = priority_queue<T>;
template <class T>
using MinHeap = priority_queue<T, vector<T>, greater<T>>;
template <class T, class F = less<T>>
void sort_uniq(vector<T> &v, F f = F()) {
  sort(begin(v), end(v), f);
  v.resize(unique(begin(v), end(v)) - begin(v));
}
const int N = 1e6 + 5;
int a[N], mp[N];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int64_t n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (int)(n); i++) cin >> a[i];
  int64_t tot = 0;
  memset(mp, -1, sizeof(mp));
  for (int i = 0; i < n; i++) {
    int last = mp[a[i]];
    int64_t x = (int64_t)(i - last) * (n - i);
    tot += x * 2 - 1;
    mp[a[i]] = i;
  }
  cout << fixed << setprecision(12) << (double)tot / (n * n) << '\n';
  return 0;
}
