Count and sum cells with a specific content

I know this is an easy one for you geeks.

I have a list of students. I wanna track their assiduity to my classes.

In the column next to their name, I write "a" when they attend, or "x" when they don't.

I need a function, able to count all "a"'s as "1" and add them up, in order for me to know how many classes each student attended.

Here's how it should look like.

Thanks in advance!