formula to add a varying suffix to a fixed number

I have an invoice with a fixed number - (9500 for sake of argument) on the first sheet of my workbook.

The invoice is payable in monthly instalments so I have the monthly invoices on the following worksheets.

I would like the invoice for the first month to be 9500a, the second sheet 9500b and so on. Is there a formula that can add to each sheet to add the alphabetic suffix to the fixed cell (and number) which is on the front sheet.