How to identify Zoho CRM contacts that have not had an activity in the last 6 months

How to identify Zoho CRM contacts that have not had an activity in the last 6 months

I am using Zoho CRM Professional Edition and am trying to find a way for the system to help me identify which contacts I should proactively contact as I have not had a recent interaction with them. 

I have tried to create a report showing contacts who's last activity (i.e. call/event) is greater than x months ago (i.e. 6 months) but can't workout how to specify this criteria.

I appreciate any help/tips.

Regards,

Zoe