Prevent users from entering alphabet!

Hi,

I have a doubt, how could i prevent the users from entering a alphabet in a cell ,like people do in java script.

example : there is a field called Quantity, since the quantity field should contain only numbers, i have done this by using data validation (i.e) the user can enter alphabets and it shows the error only after the user enters it.
but i dont want anything to be entered except numbers.

Please help!!!

Thanks