A figure is made from a large semicircle with radius 10 cm. A smaller semicircle with radius 5 cm is removed from it, and another semicircle with radius 5 cm is added to it, as shown. What is the area of the shaded region in cm$^2$?
Diagram Description: A large semicircle with diameter AB of length 20, centre O. The region is the area of this large semicircle. From this, a smaller semicircle with diameter AO (length 10) is removed. To this, a semicircle with diameter OB (length 10) is added on top. Effectively, it's a shape known as a lune, but described as a composite shape. The final shaded area is equivalent to the area of the large semicircle.