How to add user entered values dynamically in same page without redirecting to another?

I've created four text boxes(val1, val2, val3, total) in a web form. If user enters a value 20 in textbox1 the "total" should be 20, if user enters second value as 30 then total should be 50.. but should not use button redirect method. the answers should display in same page?? Can any one provide me the code. Thanks in advance.