You want to use this formula to calculate a mean average:

[[ (VAR1 + VAR2) / 2 ]]

Division is higher priority than addition so that operation happens first. You need to first sum the components and then divide that sum by the number of components. To specify this in code, as well as standard algebra, you wrap your summation in parentheses. Think of this as specifying it as “one group” or the result of the addition as “one value” within the parentheses. The computer will do just that–add add the numbers within the parentheses and divide that value by 2.