Sub total wrong in crystal report

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CrystalReports

I try to get sub total .First i create running field and select amount field from table and save it with name e.g.#stax_amount and drag this in field in crystal report then after this i write these 2 shared formulas



how to solve it

What I have tried:

formula 1
shared a as number
shared b as number
a={#stax_amount}
b=b+a
formula=a
a=0


formula 2
shared b as number
formula=b
b=0
then i drag formula 2 in total field but when i run report then total display not correct