Validate minimum order value from a form.

I have made a simple form with WA form tool, which has two products that the user can select number from. The products have different prices.
I want the form to validate the total amount (from both products). If it is less than 100, it should not be possible to send the order and a message should say that the minimum order value is 100.
How to create this?
Since this is a common way, there should be a pretty easy way to do this - or?

You would have to write a custom javascript function to do this client-side.

Server side with Server Validations is a bit easier... you could just add the two numbers and do number validation.

Unfortunately, I do not know enough javascript to write this code.
However, I have come up with a solution, but I am not happy with it. It is based on a javascript that I found, which sums the two fields in a hidden field. Then all data is sent to a confirmation page. This page retrieves the data from the first page using Form Data. Then the Show Region Utility is used, which examines whether the hidden field is below 100. In that case, an error message is displayed, and the send button cannot be pressed. The problem with this is that it requires two pages, and it feels a bit cumbersome..
How do you suggest this can be done?

With server validation, you can simply use the plus character to add two field values in php. Then use server validation to validate the result. If you attach the page I might be able to be more specific.

Sure, I need all help I can get :)
The attachment is not the file I talked about above, which I am not happy with. This is created with the WA form tool.

The easiest solution is something like this

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