#include <bits/stdc++.h>
using namespace std;
template <class X, class Y>
void amax(X& x, const Y& y) {
  if (x < y) x = y;
}
template <class X, class Y>
void amin(X& x, const Y& y) {
  if (x > y) x = y;
}
const int INF = 1e9 + 10;
const long long INFL = 1e18 + 10;
const int MAX = 1e6 + 10;
int n;
int a[MAX], prv[MAX], pos[MAX];
long long gt[MAX * 4], lazy[MAX * 4];
void update(int id, int l, int r, int u, int v) {
  if (v < l || u > r) return;
  if (u <= l && r <= v) {
    lazy[id]++;
    gt[id] += (r - l + 1);
    return;
  }
  int mid = (l + r) / 2;
  update(id * 2, l, mid, u, v);
  update(id * 2 + 1, mid + 1, r, u, v);
  gt[id] = gt[id * 2] + gt[id * 2 + 1] + lazy[id] * (r - l + 1);
}
long long query(int id, int l, int r, int u, int v) {
  if (v < l || u > r) return 0;
  if (u <= l && r <= v) return gt[id];
  int mid = (l + r) / 2;
  return query(id * 2, l, mid, u, v) + query(id * 2 + 1, mid + 1, r, u, v) +
         lazy[id] * (min(r, v) - max(u, l) + 1);
}
void process() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n; i++) {
    prv[i] = pos[a[i]];
    pos[a[i]] = i;
  }
  long double sum = 0;
  long double choose = (long long)n * n;
  for (int i = 1; i <= n; i++) {
    update(1, 1, n, prv[i] + 1, i);
    sum = sum + (query(1, 1, n, 1, i) * 2 - 1) / choose;
  }
  cout << fixed << setprecision(10) << sum << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  process();
}
