Simple Question - Please Help!
 

I have a column of data that contains a 7 digit number in each row. I need to insert a hyphen after the 2nd number for formatting purposes for the entire column. Any easy way to do it? I am getting tired of the copy and paste. Example below

As it looks now:
990000
990001

How I need it to look:
99-0000
99-0001

Thanks.