Answer
non-invertible.
Work Step by Step
We know that the determinant of a matrix \[
\left[\begin{array}{rr}
a & b \\
c &d \\
\end{array} \right]
\]
is $D=ad-bc$.
Also, a matrix is non-invertible if and only if $D=0$.
Hence here $D=(-1)(-1)-1\cdot1=1-1=0$, thus it is non-invertible.
