Contour lines from AutoCAD to QGIS: how to automatically assign the right value to the Z

I have a plate of contour lines made on AutoCAD (contour lines do not have a Z). The Z is indicated only for some curves but in a text format.
If you import the table in QGIS the situation is this (the points correspond to where the Z is indicated):

My question is: knowing that the contour lines shown are every 20 meters is there a way to automatically assign the right value to each curve having only this little information?
I thought about merging attributes between the layer of the contourlines and the text layer, but this would solve the situation for only a few curves.