Drop numbers in list with condition in python

I am trying to drop numbers located between consecutive numbers in list. Example:

Form

[1, 3, 5, 6, 7, 8, 9, 11, 12, 13, 18]

To

[1, 3, 5, 9, 11, 13, 18]

You'll see that [6, 7, 8] and [12] are dropped

You can try itertools.groupby:

from itertools import groupby

lst = [1, 3, 5, 6, 7, 8, 9, 11, 12, 13, 18]

out = []
for _, g in groupby(enumerate(lst), lambda k: k[0] - k[1]):
    g = list(g)
    out.append(g[0][1])
    if len(g) > 1:
        out.append(g[-1][1])

print(out)

Prints:

[1, 3, 5, 9, 11, 13, 18]