nginx redirect from web root to a and if 404 then try b

Question

I am trying to amend my nginx config to redirect from the web root ("/") to /dir-a and then if this returns a 404 redirect to /dir-b. I'm not entirely sure how or if this is possible though.

This is what I have so far...

    location = / {
            rewrite "/" /dir-a permanent;
    }

But this returns the 404 page so I'm not sure where to go from here. I need it to try dir-b instead.