Formula: Start new number where last number stops in a range

Sorry to hear so, but ... it works. See attached file.
The formula is in column B
as I wrote - you gave no clue why
2387-115 shall be followed by 2567-756
From the data in the sheet it could be only noticed that there sholud be some change (values in row 2)

So it works as I tried to describe: as new events are written into column A there comes such moment when the range previous range is exhausted, Operator writes manually first number RRR-XXX in Bn cell
and when new name is entered in An+1 cell the cell Bn+1 shaows new ticket number RRR-XXX+1

Try writing Ice-yachting in A17, 4578-327 will appear in B17. Then write write Wrestling in A18 and 4578-328 will appear.
If we assume that it is already after current range, just write in B18 first ticket from new range, like 1234-432*
Then when you write in A19 Short-track in B19 will appear 1234-433.

That's it


* If I were operator I would check the whole pile of new tickets and write in row 2 what ticket numbers are available in current range.