#include <bits/stdc++.h>
using namespace std;
using vll = vector<long long>;
using vvll = vector<vll>;
using vpll = vector<pair<long long, long long> >;
using vld = vector<long double>;
using vb = vector<bool>;
using vd = vector<double>;
const int inf = (int)1e9 + 7;
const long long llinf = (long long)1e18 + 7;
void solve() {
  cout << fixed << setprecision(6);
  long long n;
  cin >> n;
  long long a[n + 1];
  vvll v((long long)(1e6 + 5));
  for (long long i = 1; i <= n; i++) {
    cin >> a[i];
    v[a[i]].push_back(i);
  }
  long long ans = 0;
  for (long long i = 1; i <= 1000004; i++) {
    for (long long j = 0; j < (v[i].size()); j++) {
      long long l, r;
      if (j == 0) {
        l = v[i][j] - 0;
      } else {
        l = v[i][j] - v[i][j - 1];
      }
      r = n - v[i][j] + 1;
      ans += l * r - 1;
    }
  }
  ans *= 2;
  ans += n;
  double val = ((1.0 * ans) / (n * n));
  cout << val << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long ttt = 1;
  for (long long i = 1; i <= ttt; i++) {
    solve();
  }
}
