$2 \left \lfloor \dfrac n2 \right \rfloor = \begin{cases}n & \text{iff $n$ is even}\\n-1 & \text{iff $n$ is odd}\end{cases}$
So, $n - 2 \left \lfloor \dfrac n2 \right \rfloor = \begin{cases}0 & \text{iff $n$ is even}\\1 & \text{iff $n$ is odd}\end{cases}$
PS: The question has a typo. It says $\left [\dfrac n2 \right ]$ instead of $\left \lfloor \dfrac n2 \right \rfloor$. Let me fix it. :)