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?