Java script at server side

Question

Hi,
On button click i am  doing some calculation if it returns true i am redirecting to other page.
Now my question is if it returns false then i want to show a pop message "Do you still want to continue?".
How can we do this using java scripts or is it possible using custom validation in asp.net.
Note : For calulation i need to do in .cs file cannot do in aspx page.
Thanks