Counting unique values

Hi,

I am trying to get a formula whereby I am looking in two columns and counting unique values in one column that occur when matched against a specified criteria from another column. ie.

Me Dog
Me Dog
Me Cat
You Dog

Therefore in this example if I wanted to count how many unique values there were in column a that corresponded with the word "dog" the formula would return 2 (counting once for the two occurences of "me dog" and once for the "you dog".

Many thanks in advance,

GW