Difference between two datagrid values and display the result in a new row

I have a datagrid that displays a table from a database vertically as follows : 

  Scores1     | 75
  Scores2     | 100
  Difference  | ?

      
I have to added a new row called Difference which should display the difference between theScores1 row value  & the Scores2 row value.

(P.S : No negative values from the difference)

How to achieve this in WPF?

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