Hello, I've been trying to solve this problem: "How many generations must a person go back to have at least 1000 ancestors?"

I've been trying ways to do it.. but not sure how. I know that the series goes like 2 + 4 + 8 and so on, so I know my a=2 and r=2. Don't really know where to go from there though. The question says at least 1000, so 1000 isn't a value I can plug into my equation.

Not asking for anyone to solve, maybe just point me in the right direction on how to start. Thank you!