Answer:
We usually define the cases like this as follows:
If we go upwards N units, then we add N.
If we go downwards N units, then we subtract N.
In this case, suppose that the elevator is originally on the floor K (can be any integer number larger or equal than 2)
Then the elevator goes down 2 floors.
Then now the elevator will be on the floor (K - 2)
Then the integer that represents this scenario is -2, which represents the change in the floor number where the elevator is.
