Let a sequence $b_1, b_2, \ldots$ be defined as $b_1 = 1$, $b_2 = 2$, and $b_{n+1} = b_n b_{n-1}$.  Calculate $b_{20}$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Let a sequence $b_1, b_2, \ldots$ be defined as $b_1 = 1$, $b_2 = 2$, and $b_{n+1} = b_n b_{n-1}$.  Calculate $b_{20}$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
2^{4181}
Let a sequence $b_1, b_2, \ldots$ be defined as $b_1 = 1$, $b_2 = 2$, and $b_{n+1} = b_n b_{n-1}$.  Calculate $b_{20}$
train