Calculate prototypes Item of Network interface discovery

[Frederico]

How i can make a sum of this itens ??

On server i have:
eth0
eth1
eth1.100 (Vlan 100)
eth1.101 (Vlan 101)
eth1.102 (Vlan 101)

I need sum vlan traffic.

I tried create on host:

net.if.in[soma_link_operadora]
last("net.if.in[eth1.100,bytes]")
+last("net.if.in[eth1.101,bytes]")
+last("net.if.in[eth1.102,bytes]")

but fail .... Any tips ?