Challenge Your Brain 1

In trapezoid ${ABCD}$ with $\overline{BC}\parallel\overline{AD}$ , let ${BC}$ = ${1000}$ and ${AD}$ = ${2008}$ . Let $\angle{A}$ = $37^{\circ}$ , $\angle {D}$ = $53^{\circ}$ , and ${M}$ and ${N}$ be the midpoints of $\overline{BC}$ and $\overline{AD}$ , respectively. Find the length ${MN}$ .