it's their basis - cost and accumulated depreciation. cost should have included any improvements prior to the gift. you get to add any you made. you are also responsible for depreciation recapture that was allowed or allowable during the period they owned it. here's a prior thread on the issue towards the end.
https://ttlc.intuit.com/community/investments-and-rental-properties/discussion/tax-on-giving-rental-...
now if this has been messed up such as parents not taking depreciation on the improvements they made or perhaps a wrong assumption by you after they gifted it you probably should consult a tax pro.
