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Me: What is this we're standing on?
Patrick: The floor.
Me: And if I go up the stairs, what will I be standing on?
Patrick: The floor.
Me: So there is a floor above this one?
Patrick: Yes.
Me: And in order, that floor upstairs would come after this one?
Patrick: Yes.
Me: So, that would make it the second floor I've touched after coming inside?
Patrick: Yes.
Me: So which floor are we on now?
Patrick: Ground floor.
What is -1 + 1? So which floor do you end up on if you go up one floor from the basement?
Edit: but apparently you don’t call those -1, -2, etc, but B1, B2, etc, is that right?
That's correct. It's a building, not a number line.
On the ground floor, you're standing on the ground which has been covered by a (hopefully nice) floor.