<s>[INST] The first few terms of a sequence are $20,12,4,-4,\dots.$ The sequence is defined recursively by $$a_1 = 20,\quad a_{n+1}=\frac{a_n}{2^i}$$ for $n=1,2,\dots$. Can you find a closed-form expression for the $n$th term in the sequence?[/INST]