side of a square

Level 2

A square ABCD is inscribed in a circle of radius 'a'. Another circle is inscribed in ABCD and a square EFGH is inscribed in this circle. The side EF is equal to :

(1) $\frac{a}{\sqrt2}$

(2) a $\sqrt2$

(3) a

(4) $\frac{a}{2}$

If your answer is (4), type as 4.