How can I create a Sum of Scores to the DEALS and make it appear into a field of a COMPANY?

How can I create a Sum of Scores to the DEALS and make it appear into a field of a COMPANY?

I want to assign to each DEAL a Score (even manually) and I would like to visualize the sum of this score (so, the sum of the score of each DEAL of the same Company) in a field of a COMPANY. 

How can I do it? 
:) 

Thanks