#include <bits/stdc++.h>
using namespace std;
int n;
int a[100007];
int inv[100007];
struct node {
  double val;
  int cnt;
  node() {
    val = 0.0;
    cnt = 0;
  }
  node(double _val, int _cnt) {
    val = _val;
    cnt = _cnt;
  }
};
node unite(node p1, node p2) {
  node ret = p1;
  ret.val += p2.val;
  ret.cnt += p2.cnt;
  return ret;
}
class Tree {
 public:
  node tr[4 * 100007];
  void init(int where, int IL, int IR) {
    tr[where] = node(0.0, 0);
    if (IL == IR) {
      return;
    }
    int mid = (IL + IR) / 2;
    init(2 * where, IL, mid);
    init(2 * where + 1, mid + 1, IR);
  }
  void update(int where, int IL, int IR, int pos, double add) {
    tr[where].val += add;
    ++tr[where].cnt;
    if (IL == IR) {
      return;
    }
    int mid = (IL + IR) / 2;
    if (pos <= mid) {
      update(2 * where, IL, mid, pos, add);
    } else {
      update(2 * where + 1, mid + 1, IR, pos, add);
    }
  }
  node query(int where, int IL, int IR, int CURL, int CURR) {
    if (IL > IR || CURL > CURR) {
      return node(0.0, 0);
    }
    if (CURR < IL || IR < CURL) {
      return node(0.0, 0);
    }
    if (CURL <= IL && IR <= CURR) {
      return tr[where];
    }
    int mid = (IL + IR) / 2;
    return unite(query(2 * where, IL, mid, CURL, CURR),
                 query(2 * where + 1, mid + 1, IR, CURL, CURR));
  }
};
Tree w;
void input() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]);
    inv[a[i]] = i;
  }
}
void solve() {
  w.init(1, 1, n);
  double ans = 0.0;
  double coef = 1.0 / (1.0 * n * (n + 1) / 2.0);
  for (int i = n; i >= 1; --i) {
    node ret = w.query(1, 1, n, 1, inv[i]);
    ans += ret.cnt;
    ans -= ret.val * (n - inv[i] + 1) / 2.0;
    w.update(1, 1, n, inv[i], coef * inv[i]);
  }
  w.init(1, 1, n);
  for (int i = 1; i <= n; ++i) {
    node ret = w.query(1, 1, n, 1, inv[i]);
    ans += ret.val * (n - inv[i] + 1) / 2.0;
    w.update(1, 1, n, inv[i], coef * inv[i]);
  }
  printf("%.12lf\n", ans);
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  input();
  solve();
  return 0;
}
