Sum Total Inbound and Outbound two graphs

[devilnct]

Dear,

I created two graphs (I call graph A and B) with template Interface bits/s Total bandwidth, then I created one graph (I call graph C) which is a sum of two graphs (A and B). In graph A and B, I have total inbound and total outbound, how can I sum two values total in and out in graph A & B and bring it to graph C ? I read document Variables and it only has one command line:

"|sum:([0-9]|auto):(current|total):([0-9]):([0-9]+|auto)|"

If I use "|sum:auto:current:2:auto|" the result is total in of A (or B) and total out of B (or A)
If I use "|sum:auto:total:2:auto|" the result is total all (in A+B + out A+B)

How can I sum only total in A + B and total out A + B ?

Thank you very much
Sorry for my english