Server Expression cannot be negative

Folks,

How do we write the server expression to always display 0 or positive value? The value cannot be negative.

Here is my Server Expression.

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round(sum( nvl(   branch_data.cie_vmi_ss_cust_qty,0))) + round(sum( nvl(   branch_data.cie_vd_dm_adj_qty,0))) - round(sum( nvl(   branch_data.cie_oh_qty,0))) -  round(sum( nvl(   branch_data.cie_disc_job,0)))

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Thanks for your advice.

rgds

Aqil