formula for currency conversion at AO level

hi All,

My requirement is to get 'Amount in USD' based on source currency field (e.g Amount in any currency ) and exchange rate (e.g. ZCUR) which is reading from ECC.

'Amount in USD' field should not calculate if it is already in USD.

can anyone advise, how to write formula in HANA studio 2.0.

Regards

Naani