how to sum more then 1 textbox and total in another textbox?

Question

i have 33 textbox viz. txt1,txt2........txt33 etc. default value of all the textboxes os 0.

and one textbox i have which is txttotal.

 

when user put a number in any of the textbox automatically the sum should be calculated in txttotal. 

and txttotal textbox should bve non editable.