How to clear NaN error in the app??

Hey folks, 
I have been working on algebraic expressions recently, the correct answer is shown on my system but if I upload the same on the application there will be a NaN error. I also got to know that if I click on "Enter" button from my keypad in my Mobile and then click on calculate button , I am getting the answer (i.e, string is getting converted into number when the "Enter" button is clicked).
How Do I get the answer on my first shot when I am clicking on calculate button directly (Without "Enter" button). I will be grateful for any help.

Here is my storyline file for the reference.