#include <bits/stdc++.h>
using namespace std;
vector<long long> f1, f2, f3;
long long query(long long i, vector<long long> &f) {
  double sum = 0;
  while (i > 0) {
    sum += f[i];
    i -= (i & (-i));
  }
  return sum;
}
void upd(long long i, long long k, vector<long long> &f) {
  while (i < f.size()) {
    f[i] += k;
    i += (i & (-i));
  }
}
int main() {
  ios::sync_with_stdio(false);
  cout.tie(0);
  long long n;
  cin >> n;
  long double sum = 0.;
  vector<long long> v(n);
  vector<long long> counter(n, 0);
  f1 = f2 = f3 = vector<long long>(n + 1, 0);
  for (int i = 0; i < n; ++i) {
    cin >> v[i];
    upd(v[i], 1, f1);
    upd(v[i], i, f2);
    upd(v[i], i * i, f3);
    long double v1 = query(n, f1) - query(v[i], f1),
                v2 = query(n, f2) - query(v[i], f2),
                v3 = query(n, f3) - query(v[i], f3);
    long double j = i;
    sum +=
        v1 * j * (j + 1) / (n * (n + 1)) +
        (v1 * n * n - 2 * n * v2 + v3 - v1 * n + v2) / (n * (n + 1)) -
        (1.L / (n * (n + 1)) * (v1 * j * j - 2.L * j * v2 + v3 - v1 * j + v2));
  }
  for (int i = 1; i <= n; ++i) {
    sum += ((long double)(n + 1 - i)) * i * (i - 1) / 4. / (n * (n + 1) / 2);
  }
  cout.setf(ios::fixed);
  cout.precision(15);
  cout << sum << endl;
}
