Trigger Expression when one item value is unknown

[panmarcin]

Hello,
I have Expression on my Trigger:

({host1:key[value].last()}) +({host2:key[value].last()}) +({host3:key[value].last()}) +({host4:key[value].last()} <> 1

My goal is to check if the value 1 is returned from any one of my hosts.
Problem is when one of them if off and return value unknown.

How can i check this?

Regards