I have a ticket for a 2-segment trip (e.g., RIC-PHL-EWR) but cannot fly the first segment (e.g., RIC-PHL) and plan to just fly the second (e.g., PHL-EWR). I know that the entire itinerary will be cancelled if I don't take the first segment. If I've already checked in for both before the start of the trip, though, what would happen if I try to board the plane for the second segment; when I give the gate agent my boarding pass, will it just not work then?

Thanks.