using minus in trigger expression

[zabbix]

I all, I'm working with version 1.1 and I wanna create a trigger to see when the memory available is less than 100 MB. I have this items:
-vm.memory.size[total]
-vm.memory.size[buffers]
-memory.size[cached]
and I've defined the trigger expression:
{myhost:vm.memory.size[total].last(0)}-{myhost:vm.memory.size[buffers].last(0)}-{myhost:vm.memory.size[cached].last(0)}< 100
but it seem not work.
In other word I want to see if total-buffers-cached<100
How can I do?
Someone can help me, please?
Thanks