crystal report formula

Question

Im my crystal report i ve parameter field like 1.From account 2.To account

If user select from account alone (without choose To account) then in To account field the same from account details shoud be shoen

that is if user select From account="1" and didnt chose (To Account="") then the value should be assing like(To account="1")

i ve write query in formla editor like

if({?To A/c No})="" then {GLAS_SL_TRANSACTIONS.SLTR_SLMAST_ACNO}={?From A/c No} else

{GLAS_SL_TRANSACTIONS.SLTR_SLMAST_ACNO} in {?From A/c No} to {?To A/c No}

its not working how?