Count passengers in a bus with multiple doors

[jhergon]

When a bus has multiples doors, it is difficult to count the quantity of passengers hop in and hop off the bus.
Let's suppose we count the passengers get in and get out in one door. The passenger could enter in a door and exit in a different one. For this reason the counter could go negative.
For this reason, the counter could allow negative numbers meaning the previous capacity was higher and adjust this negative value by increasing previous values.