What should I do if the builder is not giving possession?

Hi Members, I had purchased in flat in Pune in May 2014. The builder had promised the possession will be given to use in June 2016. In the agreement for his safety he mentioned that the possession will be given by December 2016. The builder has not given the possession to me till date. The builder received the OC for their site date 31st March 2017. This was notified to me in April and they had asked us to go over the various checklists for claiming the possession of the flat. This was delayed for another two - three months. In August I approved the final checklist and have recently made the final payment. 100% amount has been paid for the flat.I have made the payment of 81lacs including maintenance, parking, everything. During the construction period, there were various payments which we had to make as per a schedule of construction. After making payment of almost 50% of home amount, my certain payment installments were delayed due to financial constraints. However, I had been clearing up the amount with a delay of approx 3months. The builder was charging me 18% interest for the amounts delayed. Today the builder says that the accumulated interest is 8.83 lacs which I need to pay in order to take possession of the flat. As per calculation on the amount I have paid 77 Lacs and 18% interest and considering a minimum delay of 6 months, the builder needs to pay me approx 7lacs. What should I do in this situation? How do I resolve this conflict and get the possession of my flat?