What is the units digit of $3^1 + 3^3 + 3^5 + 3^7 + \ldots + 3^{2009}$?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the units digit of $3^1 + 3^3 + 3^5 + 3^7 + \ldots + 3^{2009}$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
3
What is the units digit of $3^1 + 3^3 + 3^5 + 3^7 + \ldots + 3^{2009}$?
train