You get to keep only enough to maintain a very modest lifestyle in a low-cost-of-living area, the rest of it has to go towards improving the world in some way.
Edit: Given the previous rules that you must maintain a very modest lifestyle in a low-cost-of-living area, would you rather choose to opt out and not have the money at all?
Invest it, assuming 5% return, spend the interest money of 250 million per year on feeding the hungry, educating kids, and helping animals. Somewhere along the way, buy a couple of beers for myself.