This looks like a Math question, actually it is an Epiphany tradition called Chalking the door.
Some variations also put “20*C+M+B+19”, where the star stands for the star of Bethlehem.
CMB can also be the abbreviated form of the Latin blessing “Christus mansionem benedicat“, which means, “May Christ bless this house”.
The LaTeX code for it can be:
20\dagger C\dagger M \dagger B\dagger 19