Calculations in Webforms

Good day. The task is as follows. In a webform that serves as an order form, it is necessary to multiply two values and display the result in a third field, preferably without a submit button. I have a rough idea of how to perform the multiplication; I will likely need to write PHP code for calculations and output in the webform template, but how can I make it auto-submit? Additionally, the result needs to be checked. That is, if it reaches a certain value, the discount size should be calculated, displaying a field for the discount and the final result. How can I implement this check and calculation?