How to count only yes values from a yesno variable

[Gabi1]

Hi, 

I am a total newbie and I have one very basic question.

I need to calculate the rate of people that made a purchase. There is a variable for this with yes/no values. 

I need to divide only the count of “yes” of the latter variable by the total number of visitors (which I have).

The question is, how can I get the count of “Yes” only from the purchase variable?

What would be sort of an alternative of countif in Looker? I’ve tried conditional sum, but it does not work.