Uber operates an airport shuttle service.

The city is represented as an n × n grid:

0 → empty road

1 → rider waiting

-1 → blocked cell

Movement Rules:

Start at (0,0)

To airport (n-1,n-1) → move only right or down

Return to hub → move only left or up

Riders collected become 0

If no valid path exists → return 0

Goal: Maximize riders collected during full round trip.

Function Signature int maxRiders(vector<vector<int>> city);