How can I fix Calculated Item Error?

[Afateminejad]

Hello Everyone

In one of the equipment that I want to monitor, there are two items :
1- vm.memory.free[memFree.0]
2- vm.memory.total[memTotal.0]

I want to simply calculate the percentage of free memory using the calculated item.So I created a calculation item and wrote this expression in the formula section:

last ("vm.memory.free[memfree,0]" / last("vm.memory.total[memtotal,0]") *100

But after the test, an error message appears like this:
Cannot evaluate function: item "/n-sw-ad-003/vm.memory.free[memfree,0]" does not exist at "last(//vm.memory.free[memfree,0]) / last(//vm.memory.total[memtotal,0]) *100"

I changed the formula as follows:
last("vm.memory.free[memfree,0]") / last("vm.memory.total[memtotal,0]") *100


And this time the following error message is displayed:
Cannot evaluate function: invalid first argument at "last("vm.memory.free[memfree,0]") / last("vm.memory.total[memtotal,0]") *100"

I would be grateful if you could help me