Selecting chats where no engagements got a message from agents

Hi, 

I am trying to identify which chats have been actually replied by our agents and which ones are not. For that I have created a metric that counts all those chats where there were at least one replied from my human agents (excluded chatbot) by using  VALUE(Engagement agent messages)>=1.

The problem is that when i want to get all of those where there was not a message the filter dont work well as some engagements might have  messages but some others havent, within the same chat. So if I use  VALUE(Engagement agent messages)<1 will give me all of those where at least 1 engagement got not agent messages, regardless if there was any other engagement with messages. 

Any idea about how to count those that havent been properly answered at any point? 

Thanks