number of unique values in the report total only

number of unique values in the report total only

I have a standard report with managers and their transactions with clients during a month. What I need is to add a total number of (unique) clients processed this month. It is easy to add such a column using Aggregate formula but I don't need the column, only a total number of unique clients.
What is the easiest way to do it?

Thanks in advance!
Ph