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I was solving this equation:- $$key=(\sum_{K=0}^n\frac{1}{a^K})\mod m$$

Given

$$ 1,000,000,000 < a, n, m \; < 5,000,000,000 $$ $$ a, m \; are \;coprime $$

I solved it bruteforcely but it won't work in the given constrains so I need a faster algorithm or is there is something I can notice to make the formula easier to solve ?

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