A city government is considering a proposal to build a footbridge over a waterway that will benefit the residents and firms situated in the neighborhoods adjacent to the footbridge on both sides of the river. If the demand for this public good for each of \(n=20,000\) identical individuals in the two neighborhoods is \(Q=500-0.5 p_{i}\), where \(p_{i}\) is each individual's willingness to pay for the footbridge, what is the social demand curve, and how much would the residents be willing to pay collectively for the footbridge? A