Note that there is a parse difference between -1 and -(1) aka - 1:

?- X= -1, write_canonical(X), integer(X).
-1
X = -1.

?- X= -(1), write_canonical(X), integer(X).
-(1)
false.

?- X= - 1, write_canonical(X), integer(X).
-(1)
false.

However, in the is/2 expression, this doesn't matter:

?- X is -(1) + 1.
X = 0.