#include <bits/stdc++.h>
using namespace std;
const long long N = 100010;
long long n, a[N];
double bit[N];
inline void add(long long x, double val) {
  for (; x <= n; x += x & -x) bit[x] += val;
}
inline double ask(long long x) {
  double sum = 0;
  for (; x; x -= x & -x) sum += bit[x];
  return sum;
}
int main() {
  scanf("%lld", &n);
  double tt = 0.0;
  for (long long i = 1; i <= n; i++) {
    scanf("%lld", a + i);
    tt += (ask(n) - ask(a[i]));
    add(a[i], 1.0);
  }
  double ans = 0.0;
  for (long long i = 1; i <= n; i++) {
    ans += (0.25 * i * (i - 1) + tt) * (n - i + 1);
  }
  memset(bit, 0, sizeof(bit));
  double cur = 0.0;
  for (long long i = 1; i <= n; i++) {
    double tmp = ask(n) - ask(a[i]) + cur;
    ans -= tmp;
    add(a[i], i);
    cur = tmp;
  }
  ans = ans / n / (n + 1) * 2.0;
  printf("%.12lf\n", ans);
}
