Cost of Acquisition of Listed Equity Share

I purchased a share in 1990 and got physical share certificate. In November 2017, I Dematerialized the share. As per the provisions of the Financial Budget of 2018, if a seller makes long term capital gain of more than Rs. 1 lakh on sale of equity shares or equity oriented units of mutual fund, the gain made will attract a capital gains tax of10% and the benefit of indexation will not be available to the seller.
As I told, shares purchased in 1990, so the cost of the shares would be the value on 31st January 2018 or the original cost ? Asking this query as the capital gain would be more than 1 lacs