28. In a square $A B C D$ with a side length of 1 cm, draw quarter circles with a radius of 1 cm, centered at $A, B, C, D$, intersecting at points $E, F, G, H$, as shown in the figure. The perimeter of the shaded area in the middle is $\qquad$ cm.
Answer: $\frac{2}{3} \pi$