Essbase function to get a summation of all branches

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Hi 

We are transferring our data from BSO to ASO cube for aggregation.

However there is a requirement to have a interim aggregation in the BSO cube, for a certain allocation formula.

Ex: we enter a certain value at the total branch level, and we need it to be allocated branch wise according to a certain basis. Basis is again "X" at total branch level divided by "Y" Branch Wise. 

To obtain X at total branch level, we need to aggregate the data, but we do not want to perform an aggregation in the middle. 

So my requirement is to use an essbase function which calculates summation of the total branches within the formula