\left[\begin{array}{ccc}
1&0&0\\
0&-1&0\\
0&0&1
\end{array}\right] \cdot
\left[\begin{array}{ccc}
1&0&0\\
0&-1&0\\
0&0&-1
\end{array}\right]
=\left[\begin{array}{ccc}
1&0&0\\
0&1&0\\
0&0&-1
\end{array}\right]