Query on tax to be paid on redemption of preference shares

Hi, 

I was holding equity shares of Cairn India.  Cairn India was merged into Vedanta.  So I received 1 equity and 4 preference shares of Vedanta for a single share of Cairn India.  

The preference shares have been now redeemed by Vedanta. I am still holding the equity shares of Vedanta. 

Query is what should be the cost of acquisition for the preference shares.?