Number variable error when value is reset to zero

Hello!

I can't quite figure out what's wrong with my settings. I am creating a Stock shares calculator with a bunch of number variables and conditions.

The error happens when doing this sequence of actions:

1. I choose my number of shares and enter a number in the price field, calculate, then click Buy. (no problem there)

2. But when I click the "clear button", then try to increase the Shares to say 1000, put a number in the Price field, calculate, then Buy -- the newly bought shares doesn't correctly add up to the current number of shares.

This only happens when I click the Clear button. I attached the .story file.

Here is also a Screencast link to Articulate Review:

https://360.articulate.com/review/content/ec51280f-c577-4c8d-814d-d2d0a03efd7c/review

Hope to get some help! Let me know if I wasn't clear, English isn't my mother tongue. :)