A square with sides of length $6 \mathrm{~cm}$ is given. The boundary of the shaded region is defined by two semi-circles whose diameters are the sides of the square, as shown.
The area of the shaded region is __________ $\mathrm{cm}^2$.
- $6 \pi$
- $18$
- $20$
- $9 \pi$
