Your money, properly invested, should grow, even if you invest conservatively.

So you don't need to draw down your principal to pay your living costs.

Let's say you spend $100K per year, and you can earn 10 percent per year, then you need $1 million.

But you don't even need that much, because there's no reason to die with $1 million in the bank.

So it's probably some kind of calculus problem, an integral or a derivative, or something.

And maybe you can do better than 10 percent! ;-)