LTCG - shares purchased on OTCEI & sold on NSE

I had purchased shares of Galaxy Surfactants on the erstwhile Over The Counter Exchange of India in 1997. I had paid all the statutory duties when buying the shares.
The Over The Counter Exchange of India was shut down by SEBI in 2015.
Galaxy Surfactants was subsequently listed on NSE via IPO on 8th February 2018.
If I sell my Galaxy Surfactants shares on NSE with payment of STT how will the LTCG be calculated?

Thanks