#include <bits/stdc++.h>
using namespace std;
const int maxn = 111111;
long long tree[maxn * 4];
void Insert(int o, int l, int r, int k, int v) {
  if (l == r) {
    tree[o] = v;
    return;
  }
  int mid = (l + r) >> 1;
  if (k <= mid)
    Insert(o * 2, l, mid, k, v);
  else
    Insert(o * 2 + 1, mid + 1, r, k, v);
  tree[o] = tree[o * 2] + tree[o * 2 + 1];
}
long long Query(int o, int l, int r, int L, int R) {
  if (L <= l && r <= R) return tree[o];
  int mid = (l + r) >> 1;
  long double ans = 0;
  if (L <= mid) ans += Query(o * 2, l, mid, L, R);
  if (R > mid) ans += Query(o * 2 + 1, mid + 1, r, L, R);
  return ans;
}
int n, x, a[maxn], f[maxn];
int main() {
  cin >> n;
  long double ans = 0;
  for (int i = 1; i <= n; i++) cin >> a[i], f[a[i]] = i;
  for (int i = 1; i <= n; i++) {
    ans += Query(1, 1, n, a[i], n);
    Insert(1, 1, n, a[i], 1);
  }
  ans *= ((long long)n * (n + 1));
  memset(tree, 0, sizeof(tree));
  long long last = 0;
  for (int i = 1; i <= n; i++) {
    ans -= (last + Query(1, 1, n, a[i], n));
    last = last + Query(1, 1, n, a[i], n);
    Insert(1, 1, n, a[i], f[a[i]] * 2);
  }
  for (int i = 1; i <= n; i++)
    ans += ((long long)(n - i + 1) * i * (i - 1) / 2);
  ans /= ((long long)n * (n + 1));
  cout << setprecision(15) << ans << endl;
}
