Four identical circles are drawn in a square such that each

Four identical circles are drawn in a square such that each circle touches two sides of the square and two other circles (as shown in the figure below). If the side of the square is of length 20 cm, what is the area of the shaded region?

(A) 400 100
(B) 200 50
(C) 100 25
(D) 8
(E) 4

Could not understand the solution, need help.