#include <bits/stdc++.h>
using namespace std;
const int N = 1e6;
int lst[N + 10];
long long tree[N * 4 + 10], lazy[N * 4 + 10];
void push(int v, int tl, int tr) {
  int tm = (tl + tr) / 2;
  tree[v * 2] += 1ll * (tm - tl + 1) * lazy[v];
  tree[v * 2 + 1] += 1ll * (tr - tm) * lazy[v];
  lazy[v * 2] += lazy[v];
  lazy[v * 2 + 1] += lazy[v];
  lazy[v] = 0;
}
void update(int v, int tl, int tr, int l, int r) {
  if (l > r) {
    return;
  }
  if (tl == l && tr == r) {
    tree[v] += r - l + 1;
    lazy[v] += 1;
  } else {
    push(v, tl, tr);
    int tm = (tl + tr) / 2;
    update(v * 2, tl, tm, l, min(r, tm));
    update(v * 2 + 1, tm + 1, tr, max(l, tm + 1), r);
    tree[v] = tree[v * 2] + tree[v * 2 + 1];
  }
}
int main() {
  long long sum = 0;
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    int a;
    scanf("%d", &a);
    update(1, 1, n, lst[a] + 1, i);
    sum += tree[1];
    lst[a] = i;
  }
  sum -= n;
  sum *= 2;
  sum += n;
  double all = 1.0 * n * n;
  printf("%.15lf\n", sum / all);
}
