Blessing our 'Home'

Fr John gave us some blessed chalk so that we can bless our school (which is like our home) with a traditional blessing for the new year.
We have put 20+C+M+B+21 on the wall to the entrance to the school. As we wrote this we said the blessing;

'The Three Wise Men, Casper, Melchior and Balthazar followed the star of God's Son who became human two thousand years ago. May Christ bless our home and remain with us throughout the new year. Amen'

We then said the the prayer together;
'Visit, O Blessed Lord, this home with gladness of your presence. Bless all who live or visit here with the gift of your love; and grant that we may manifest your love to each other and to those whose lives we touch. May we grow in grace and in the knowledge and love of you; guide, comfort and strengthen us in peace, O Jesus Christ, now and forever. Amen'

What does the inscription mean? 20+C+M+B+21

The letters have two meanings. First, the Magi - Casper, Melchior & Balthazar - who came to visit Jesus in His first home.

They also abbreviate the Latin phrase, Christus mansionem benedicat: "May Christ bless this house."​