Progressive numbering with variables

Hello everybody. Hope you can help me.

I'm looking for a compound formula that allows me to result in an incremental number based on two variables.
Column A -> I have a series of dates they can be also duplicated
Column B -> I have values 0 or 1
If the cell in column B equals 0 the result will always be -1, this is simple.
Therefore:

Cell C2 = -1 because B2 = 0

Cell C1 = 1 because cell B1 = 1 and in the set of column A where the dates are May 25 this is the first row of this range

Now comes the problem that I can't solve:
Cell C3 = 2 because B3 = 1 and A3 is part of the "May 25"'s range and is the second occurrence in which this two rules are verified.

And so on.

Can someone help me?

Thank you for your help,
Michele.