Get the number of disks attached to VM in vRO

Hi Experts,

I am new to vRO and I am struggling since 2 days to find a way to get the number of virtual disks attached to a VM thru scriptable task in a newly created workflow.

In guest VM, it is very simple powershell cmdlet       [ (get-disk).count] .. But I want to do this simple task using scriptable task method using javascript..

I am trying to run the task as below:

var disknum=vm.VcVirtualMachineConfigSummary.numVirtualDisks

System.log(disknum)

where vm is an input with the type VC:VirtualMachine... when I run the workflow, I dont  get any output.. (undefined)

I am not sure what exactly I am doing wrong.. can anyone please guide me here???