Service tax on freight

we paid the freight of Rs. 20000/- to transporter for delivery of goods & we charged in bill from party Rs. 30000/- against freight. Now under RCM service tax required to paid by us. Problem is that on what amount service tax should be deposited i.e. on Rs. 20000/- charged by the transporter for us or Rs. 30000/- charged by us from party ...???