A circle is inscribed inside a trapezoid $ABCD$ ($AB || CD$). $M, N, P, Q$ are the midpoints of the sides $AB, BC, CD, AD$, respectively. If $AD = d$ and $BC=c$, express the perimeter of $MNPQ$. (Answer: $c + d$)
I think that $P = \frac {AC}{2} + \frac {DB}{2} + \frac {AC}{2} + \frac{DB}{2} = AC + DB.$ I do not know how to express it in a different way using $d$ and $c$. Thank you in advance!