#include <bits/stdc++.h>
using namespace std;
const long long md = 1e9 + 7;
long long a[100005], n;
void add(vector<long long> v[], int l, int k) {
  for (int p = 0, lol = l - 1; p < 17; lol /= 2) v[p++][lol] += k;
}
long long sum(vector<long long> v[], int l, int r) {
  long long i = 0, ans = 0;
  for (l--, r--; l < r; ++i) {
    if (l % 2) ans += v[i][l++];
    if (r % 2 == 0) ans += v[i][r--];
    if (l >= r) break;
    l /= 2, r /= 2;
  }
  if (l == r) ans += v[i][l];
  return ans;
}
int main() {
  ios_base::sync_with_stdio(0);
  long double ans = 0, den, ad = 0, sub = 0;
  cin >> n;
  vector<long long> invtree[17], sumtree[17];
  for (int i = 0; i < n; ++i) cin >> a[i];
  for (int i = 0; i < 17; ++i) {
    invtree[i].resize(n / (1 << i) + 2, 0);
    sumtree[i].resize(n / (1 << i) + 2, 0);
  }
  for (int i = n; i--; add(invtree, a[i], 1)) ans += sum(invtree, 1, a[i]);
  den = 1.0 / n / (n + 1);
  for (int i = n; i--;) {
    ad += (long double)sum(sumtree, a[i], n) * den * (i + 1);
    sub += (long double)sum(sumtree, 1, a[i]) * den * (i + 1);
    add(sumtree, a[i], n - i);
  }
  ans += ad - sub;
  cout << fixed << setprecision(14) << ans;
}
