#include <bits/stdc++.h>
using namespace std;
inline bool isvowel(char c) {
  c = tolower(c);
  if (c == 'a' || c == 'e' || c == 'i' || c == 'y' || c == 'o' || c == 'u')
    return 1;
  return 0;
}
const double eps = 0.000001;
const long double pi = acos(-1);
const int maxn = 1e7 + 9;
const int mod = 1e9 + 7;
const long long MOD = 1e18 + 9;
const long long INF = 1e18 + 123;
const int inf = 2e9 + 11;
const int mxn = 1e6 + 9;
const int N = 1e6 + 123;
const int M = 22;
const int pri = 997;
const int Magic = 2101;
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
int n, lst[N], a[N];
int main() {
  cin >> n;
  long double ans = 0;
  for (long long i = 1; i <= n; i++) {
    cin >> a[i];
    ans += (i - lst[a[i]]) * (long double)(n - i + 1);
    lst[a[i]] = i;
  }
  ans *= 2;
  ans -= n;
  cout << fixed << setprecision(11) << ans / (n * (long double)n);
  return 0;
}
