Trapped inside a trapezium

A trapezium $ABCD$ ( with $AB || CD$ ) is inscribed inside a circle of centre $O$ and its diagonals $AC$ and $BD$ intersect each other at $M$ . If $OM = 2$ and $\angle AMB = 60 ^\circ$ , then find the difference between the parallel sides.

If your answer is $p\sqrt q$ , where $p$ and $q$ are positive integers with $q$ square-free, submit $p+q$ .