Simple + calculation in items (?)

Setup, Configuration and Use Items & Sitemaps
2

You would need a rule if the inputs are the DecimalType states of two items, and the output is the sum of those two Number items:

items:

Number InputA
Number InputB
Number Total

rule:

import org.openhab.core.library.types.DecimalType // not needed on OH2
rule SumTwoInputs
when
  Item InputA changed or
  Item InputB changed
then
  if (InputA.state instanceof DecimalType && InputB.state instanceof DecimalType) {
    Total.postUpdate((InputA.state as DecimalType) + (InputB.state as DecimalType))
  } else {
    Total.postUpdate(Undefined)  // or UNDEF on OH2
  }
end