Subtract If Statement

I am looking for a formula that will subtract subject c from subject a because the output variable reads "x" for both rows. I want it to skip all the "y" variables so that A-C = 50 and C-E will equal 40 for example. I was playing with a nested if statement but can not get it to work.


Subject Score variable output
Subject A 100 x 50
Subject B 75 y
Subject C 50 x 40
Subject D 25 y
Subject E 10 x

Attached is the excel file in a zip file. Thanks again, any help is appreciated.

Chris