Points $A = (3,9), B = (1,1), C = (5,3),$ and $D = (a,b)$ lie in the first quadrant and are the vertices of quadrilateral $ABCD$. The quadrilateral formed by joining the midpoints of $\overline{AB}, \overline{BC}, \overline{CD},$ and $\overline{DA}$ is a square. Find the sum of the coordinates of point $D$.

Let's think step by step and output the final answer within \boxed{}.
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