What is the greatest prime factor of the sum of the arithmetic sequence $1 + 2 + 3 + \cdots + 80$?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the greatest prime factor of the sum of the arithmetic sequence $1 + 2 + 3 + \cdots + 80$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
5
What is the greatest prime factor of the sum of the arithmetic sequence $1 + 2 + 3 + \cdots + 80$?
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