Service tax (gta)

Dear Experts,

Kindly give your's advice on following issue.

An goods were purchased by our company.

Transport billty amount is Rs. 30000.

Rs. 12000 paid by the Seller Company and balance Rs. 18000 paid by the usto transporter.

Now the question is under Reverse charge at what amount we have to pay service tax on cartage?

@ Rs. 18000 which we paid

OR

@ Full Amount Rs. 30000 

Kindly advice.....

Thanks,