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?
That’s at most only 5-10% of 5 billion
What? Land, surveys, taxes, permits, land-use studies, environmental impact studies, architects, engineers, living wage construction workers, materials, faculty, staff, doctors, pharmaciststs, care providers, business, health, property, other insurances, dieticians, meals, maintenance…I want these things funded for centuries. May as well include uniforms, so parents don’t come out of pocket, lawyers, potential lawsuits…5 bn isn’t going to last long.