Gradually Add O Substract Numbers In A Column

how do I gradually add o substract numbers in a column for example if I have



total
deposito
test

120

120


80
40

77

117

7

124


4
120


i need in the test column:
if the number is in the total column the number in test is added
if the number is in the deposito column the number in test is substracted

i have this query:


SELECT gir_cantidad as total,null as deposito,'operation' as test

FROM giros inner join corresponsales on cor_corresponsal_id = gir_corresponsal_id where gir_fecha >= '11/21/2007' and gir_fecha <= '11/22/2007 23:59:59' and gir_fecha_anul is null and gir_agencia_id = 1

UNION

select null as total,paa_valor as deposito,'operation' as test from pagosagencia where paa_fecha >= '11/21/2007' and paa_fecha <= '11/22/2007 23:59:59' and paa_agencia_id= 1 order by gir_fecha

In column test i need the functionality to add and subtract the columns total and deposito

Please help