Pythonリストの不連続数を探し出す

例:
a=[1,2,4]出力3
a=[1,2,3,4,8,9,12]出力5のみ
検索:

>>> a = [1, 2, 3, 4, 8, 9, 12]

>>> set(range(a[0],a[-1]+1))-set(a)

{10, 11, 5, 6, 7}

>>> sorted(list(set(range(a[0],a[-1]+1))-set(a)))

[5, 6, 7, 10, 11]

>>> sorted(list(set(range(a[0], a[-1]+1)) - set(a)), reverse=True)

[11, 10, 7, 6, 5]

>>> sorted(list(set(range(a[0], a[-1]+1)) - set(a)), reverse=True).pop()

5