If your SharePoint 2013 Farm is just RTM, Make sure that you have installed March PU prior to the August Update as it’s mandatory to proceed with future updates.
If you aren’t having March PU, you wont be able to proceed with any later updates and below error will pop out.
“The expected version of the product was not found on the system”
KB 2817517 – SharePoint Foundation 2013
KB 2817616 – SharePoint Server 2013
KB 2817521 – Office Web Apps Server 2013
KB 2817507 – SharePoint Designer 2013
KB 2817615 – SharePoint Server 2013 with Project Server 2013
Note :
Anyway what’s the difference between PU and a CU package. Stefan Gobner has written a informative article for TechNet which I would suggest you to refer on this.
Further information, for Farm which are having Search Components and Highly available search topologies are enabled across servers refer to Kirk Stark’s Article on How to install update packages on a SharePoint farm where search component and high availability search topologies are enabled (which is well described)
Microsoft Cloud Infrastructure Technical Journal 
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