He just blurted out an equivalent answer, but it's not a
verbatim translation.  Here is how to translate it verbatim:

Substitute "N" for the words "a number:

Substitute "N/2" for the words "the quotient of N and two"

To make something less you subtract from it.
So to make it *two less* you subtract 2 from it, so
you end up with

N/2 - 2.

Edwin