#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  const int mx = int(1e6) + 1;
  vector<vector<int>> occ(mx);
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    occ[x].push_back(i);
  }
  const long long int all = 1ll * n * n;
  long double ans = 0;
  cout << fixed << setprecision(10);
  for (int i = 0; i < mx; i++) {
    int ant = -1;
    for (int j = 0; j < (int)occ[i].size(); j++) {
      long long int qnt = 2ll * (occ[i][j] - ant) * (n - occ[i][j]) - 1;
      long double prob = (long double)qnt / all;
      ans += prob;
      ant = occ[i][j];
    }
  }
  cout << ans << endl;
  return 0;
}
