Fifteen puzzle solvability, Numworks Python

[stored]

May be like this, but not so concise.

Code:

def fifteen(a):
  b = 0
  for i in range(16):
    if a[i] == 0:
      b += (i + 3) // 4
  for i in range(15):
    for j in range(i+1, 16):
      b += (a[j] < a[i]) * (a[j] != 0)
  return b