Automatically update total field without any click events happening

Hi,
 
I have two textboxes which have number values in them
I then have a total textbox where I want to display the total value of my two textboxes (e.g. textbox1+textbox2)
I want to do it so that when user inputs both of these values, the total textbox automatically updates with the new total.. I need to do this without any click of a button.
 
Does anyone know how I can do this?
 
Thanks
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