#include <bits/stdc++.h>
using namespace std;
template <typename T>
T gcd(T a, T b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
template <typename T>
T pow(T a, T b, long long int m) {
  T ans = 1;
  while (b > 0) {
    if (b % 2 == 1) ans = ((ans % m) * (a % m)) % m;
    b /= 2;
    a = ((a % m) * (a % m)) % m;
  }
  return ans % m;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  cout << fixed << setprecision(10);
  vector<int> arr;
  vector<long long int> dp(n + 1, 0);
  arr.push_back(-1);
  dp[0] = 0;
  for (int i = 0; i < n; i++) {
    int item;
    cin >> item;
    arr.push_back(item);
  }
  vector<int> lastOc(1000010, 0);
  long long int sum = 0ll;
  for (int i = 1; i <= n; i++) {
    dp[i] = dp[i - 1] + (i - lastOc[arr[i]]);
    lastOc[arr[i]] = i;
    sum += dp[i];
  }
  sum = sum * 2 - n;
  long long int divBy = (long long int)n * n;
  double ans = (sum) / (divBy * 1.0);
  cout << ans;
}
