Counting Unique Values Across 2 Columns

This seems simple enough, but I can't seem to find anything. I have two columns, C and D, with first and last names in each one respectively. I would like to count the unique values across both columns. I.e., how many different people there are. I figured out how to do one column, but due to the fact that some people have the same first name and different last names (or vice-versa), this is not particularly useful. What's the best way to do this?