Create a pop up box for user data input that auto formulates specific cells

For Excel 2010, I have two columns created for counting.  One counts up to a number and the other column counts backwards from that number to 1.  Ex. First column counts up to 50 and the second column counts backwards from 50 to 1

I want to create a pop up box when the spreadsheet is first opened where the user inputs a value (ex. 50) and it automatically fills the first and second columns appropriately.