Mapping in SAP PI

Hi Experts,

I have recently started to work on SAP PI. So I'm relatively new to sap pi. I have to design a mapping. The condition is as follows

X is the  field in source structure------>  If ‘N’ then taregt field(Y) = ‘A’ else Target field(Y) = ‘B’  also note that X field is not always be populated .

How can I accomplish this? Please help.

Thanks

Regards,

Ravi