I need a DC adapter for my midi keyboard, but I'm not sure what size or what "letter" I need to get.

I took the keyboard down to Radioshack and showed them...they didn't have the one I needed, but the lady preceeds to sell me a smaller amp adapter that "doesn't fit".

I think it is "positive polarity tip"



And then below that it says DC 9-12V

The keyboard I have is "M-audio keystation 49e" midi controller