Cannot calculate items

[kachijs]

I wanted to know how much physical memory is actually used but it seems it cannot be done.

So i found a "calculated" solution. I created txt file in C:\physicalmemory.txt with the physical memory value in bytes. Created string in zabbix config:

UserParameter=physicalmemory,type C:\physicalmemory.txt

Then i had monitored vm.memory.size[free] allready.

So i created new item with key (TOTAL MEM USAGE)

and in "Formula" field i added last("physicalmemory")-last("vm.memory.size[free]"), but this solution is not working. In host items it shows Not supported: Cannot evaluate function [last()]

any ideas ?