zabbix and cisco fec monitoring

[wojtek_z]

Hi,
I have new installation of zabbix (early i use cacti and nagios). I have a question:
how can i define item which is definition of Forward Error Correction http://www.cisco.com/en/US/tech/tk86...shtml#fec_snmp

I don't have idea how to definie two formulas:

% Correctable = [(Ec1 – Ec0)/ [(Eu1 – Eu0) + (Ec1 – Ec0) + (C1 – C0)]] * 100
% Uncorrectable = [(Eu1 – Eu0)/ [(Eu1 – Eu0) + (Ec1 – Ec0) + (C1 – C0)]] * 100

Cx = docsIfSigQUnerroreds at time x
Ecx = docsIfSigQCorrecteds at time x
Eux = docsIfSigQUncorrectables at time x

I have items with current value but how to get last value - to calculate diff between current and last ??