How to get only count(xx) in REST

I tried many things but I am unable to get only the count of documents that is belonging to a given contactId.

This is my REST: /v1/Archive/FindDocument?$select count(documentId):HideDetail:Footer&$filter=contactId=38

But not only do I get all documents on the contactId=38, but I don't get the count as a column, which I expected. Of cause RowCount works but that was not the idea.