auto item number when a cell has figure

i have a list of item on column A1 to A1000. and when I manually put a figure in column B i want an auto item number on its column C.

ex:

A (list) B (manual) C (auto item number)
APPLE 10 1
PAPAYA
ICECREAM 5 2
SALAD 8 3

i hope you understand. :-)

i attached the sample

Capture.JPG

If your table is A1:C8, input in C2:
=IF(B2<>"",MAX($C$1:C1)+1,"")
copy down till required.