#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6 + 10;
const long long mod = 1e9 + 7;
long long read() {
  long long x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') x = (x << 1) + (x << 3) + c - '0', c = getchar();
  return x * f;
}
double ans;
struct BIT {
  long long tr[N];
  long long Bmax;
  long long lb(long long x) { return (-x) & x; }
  void init(long long n) {
    Bmax = n;
    for (long long i = 0; i <= n; i++) tr[i] = 0;
  }
  long long query(long long x) {
    if (x <= 0) return 0;
    long long res = 0;
    while (x) {
      res += tr[x];
      x -= lb(x);
    }
    return res;
  }
  void update(long long x, long long w) {
    if (x <= 0) return;
    while (x <= Bmax) {
      tr[x] += w;
      x += lb(x);
    }
  }
} t1, t2;
int a[N];
int main() {
  long long n = read();
  for (int i = 1; i <= n; i++) a[i] = read();
  for (long long i = 1; i <= n; i++) {
    ans += (double)1.00 * i * (i - 1) / 4.00 * (n - i + 1);
  }
  t1.init(n);
  t2.init(n);
  for (long long i = 1; i <= n; i++) {
    long long o1 = t1.query(n) - t1.query(a[i]);
    long long o2 = t2.query(n) - t2.query(a[i]);
    t1.update(a[i], 1);
    t2.update(a[i], i);
    ans += (double)(1.00 * n * (n + 1) * o1 * 0.5 - 1.00 * (n - i + 1) * o2);
  }
  printf("%.10lf", ans / (1.00 * n * (n + 1) / 2));
}
