adjust variable - no calculation option

Hi, I would like to do some calculation using variables, but there is no option to do so. the Trigger wizard window is different from Support doc and the pictures other people posted. I attached my screenshot.

In the support doc, you can set the "Set" to "Add", but it is not available in my software. 

Here is the link to the support doc.

https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-work-with-variables#triggers

Any suggestions?

Thank you!