Row number automatication formula

Hello,

Need help with this formula:

1. I have one column for project and second column for row nr calculation. Formula should calculate number by logic: if itīs first row, then calculate 1, if itīs second row of this project previous row + 1. Problem is sometimes, I need to start not from 1 but for example from 8 (I will manually overwrite formula to 8) and next row should calculate 8 +1 =9. Added excel also.

Projekt Number
A 1 I tīs first row for this project, automatically set 1.
B 8 itīs first row for this project, automatically set 1, but it this project neet to manually change 8
A 2 itīs second row for this project, set previous number + 1
A 3 itīs third row for this project, set previous number + 1
C 1 itīs first row for this project, automatically set 1.
B 9 itīs second row of this project, set previous number + 1

Thanks.