How and what cost to show for shares sold that date back to decades in a itr

Pushkar (Article) (25 Points)

18 July 2016  
Kindly help by telling how and what cost should be shown in the ITR for the shares sold by the taxpayer in the PY that were purchased decades back although im aware that the gain is long term and exempt from any tax..How and what amount of purchases should be shown in the ITR under Capital Gains?