The sequence $(a_n)$ of real numbers satisfies for every positive integer $n$
$$a_{n} + (-1)^{n} \cdot a_{n+1} = 2^{n}$$
If $a_{1} = 0$, what is the sum $a_{3} + a_{4} + a_{5} + a_{6}$?
A) 6 B) 8 C) 12 D) 16 E) 20