#include <bits/stdc++.h>
using namespace std;
int n;
int pos[100005];
pair<int, long long> ai[4 * 100005];
void update_ai(int node, int l, int r, int pos, pair<int, long long> val) {
  if (l == r) {
    ai[node] = val;
    return;
  }
  int mid = (l + r) >> 1;
  if (pos <= mid)
    update_ai(2 * node, l, mid, pos, val);
  else
    update_ai(2 * node + 1, mid + 1, r, pos, val);
  ai[node].first = ai[2 * node].first + ai[2 * node + 1].first;
  ai[node].second = ai[2 * node].second + ai[2 * node + 1].second;
}
void query_ai(int node, int l, int r, int lq, int rq,
              pair<int, long long> &ans) {
  if (lq <= l && r <= rq) {
    ans.first += ai[node].first;
    ans.second += ai[node].second;
    return;
  }
  int mid = (l + r) >> 1;
  if (lq <= mid) query_ai(2 * node, l, mid, lq, rq, ans);
  if (mid + 1 <= rq) query_ai(2 * node + 1, mid + 1, r, lq, rq, ans);
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) {
    int x;
    scanf("%d", &x);
    pos[x] = i;
  }
  double sol = 0;
  for (int i = 1; i <= n; ++i) {
    pair<int, long long> x = make_pair(0, 0);
    query_ai(1, 1, n, 1, pos[i], x);
    sol += ((double)(n - pos[i] + 1)) / n * (((double)x.second) / (n + 1));
    x = make_pair(0, 0);
    query_ai(1, 1, n, pos[i], n, x);
    sol += x.first -
           ((double)pos[i]) / n * (x.first - ((double)x.second) / (n + 1));
    update_ai(1, 1, n, pos[i], make_pair(1, pos[i]));
  }
  printf("%.20f\n", sol);
  return 0;
}
