conditional count

Hi Team,

Can some one please help me to get distinct count of "column A" if column b contains text as "positive".

Please find sample data below . I am unable to find formula.

Column A	Column B
a	positive
a	positive
a	positive
a	positive
a	positive
b	Negative
b	Negative
b	Negative
b	Negative
b	Negative
b	Negative
c	Positive
c	Positive
c	Positive
c	Positive
c	Positive
c	Positive