How can I implement "+=" operator using update/calculate conditions?

I have a number field that I want to use to keep a running sum. Say, I have a field named "Number 1", the moment I fill it in I want to take the value currently in "Running Sum" add the value in "Number 1" and store the value back in "Running Sum".  I've included a screenshot of how I am currently trying to implement the solution but I am getting very strange errors (The value in "Number 1" is added each time I select the field). 

Unfortunately creating a running total (+=) or (a = a + 1) doesn't work in JotForm. If you really need that kind of calculation you will need to get the source code for your form and implement this function in JavaScript.

How to get the Full Source Code of your Form