Auto Numbering (letter-number)

i want to make a data field with an auto numbering , but i exactly want is like this CT-1 , CT-2,CT-3 , the Ct letters must be before the autonumber , so its seems to me in that case i cant choose the dat field in my db as int and choose identity Yes cause the identity seed will be an integer only and no way to put with it a number.

so i want to make throw asp and put this datfield as a char so it can accept the(letter-number)value , so how can the asp make an autonumbering, and know what the lastnumber so it can add 1 to it.