#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  long long X = 0, w = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') w = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') X = X * 10 + c - '0', c = getchar();
  return X * w;
}
const long long N = 100000 + 11;
long long n, a[N];
struct BIT {
  long long c[N];
  inline void add(long long x, long long y) {
    for (; x <= n; x += x & -x) c[x] += y;
  }
  inline long long sum(long long x) {
    long long s = 0;
    for (; x; x -= x & -x) s += c[x];
    return s;
  }
} C, S;
signed main() {
  n = read();
  for (register long long i = 1; i <= n; ++i) a[i] = read();
  double ans = 0;
  for (register long long i = 1; i <= n; ++i)
    ans += 0.5 * i * (n - i) * (n - i + 1);
  for (register long long i = n; i; --i) {
    ans += C.sum(a[i] - 1) * (n * n + n - 2 * i - 2 * n * i);
    ans += 2 * S.sum(a[i] - 1) * i;
    C.add(a[i], 1), S.add(a[i], i);
  }
  printf("%.20lf\n", 1.0 * ans / n / (n + 1));
  return 0;
}
