autofill a textbox based on another textbox input.

I have two textboxes: Textbox1 and textbox2 on the page. the value of textbox2 equals the value of textbox1 minus 1(textbox2.text = textbox1.text - 1).
When I input textbox1 some value then I want textbox2 to be filled automatically. 
How can I do that in ASP using C#?

Thanks.
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