Need to count the same data and assign a sequence number

I have a spreadsheet (AG Orders) and Im trying to create a formula that counts how many times "DEP" is entered in column C. and then assigns that # in column K for that row. The problem that I am running into is, if I sort the spreadsheet, column K's # changes.

Currently the formula I am using is =IF($C133="DEP",COUNTIF($Y$2:$Y133,$Y133),"") which works initially as long as I never sort the spreadsheet. Once I spreadsheet, the numbers change depending on where they are on the sheet.

15-16 budget-AG.xlsm

After you copy the formula down, you can then fix the values before you sort, so then they won't change.

Hope this helps.

Pete