A bag contain $5$ white balls and $4$ black balls. One ball is drawn at a time . 
What is the probability that the balls drawn will be alternately white and black ? 

My try : 

i though $p(W)$ is white ball and $p(B)$ is black ball 

This event is not independent , its dependent. 

So used the formula 

$p(aleternative) = P(W)\*P(B|A)$

$= 5/9 \*1/2$

$= 5/18$

But this is wrong answer .. Guide me where i am getting wrong.