Formulas in Canvas Quiz

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I have been able to successfully input formulas into my Quiz (not New Quiz) using multiple variables.  I can successfully enter Eulers rule for faces, vertices, and edges  (V+F-2=E) solving for E.

However, I cannot get it to work for solving for the number of faces. I currently have e-v+2.  With random numbers generated in the quiz e = 17 and v = 7, I get the answer of -2.  The E value is set to 15-25 no decimal.  The V value is set to 5-7 no decimal.I put in e-v into the formula and get the answer -4.  I've tried using parentheses, spaces, variations of the variables(-f-2+e).  

Can someone let me know what I am doing wrong please?

Thank you.