A nice and short geometry problem

Let $ABC$ be a triangle with $\angle ABC=120^{\circ}$ . Construct an isosceless triangle $ACD$ with $\angle BDC=120^{\circ}$ . ( $D$ and $B$ are not on the same side with respect to the line $AC$ ). Find $\angle ABD - \angle BAC$ (the difference of angle measures), in degrees.