Calculate gains - Selling shares after de-merger

Respected Sir/Madam,

I sold shares of companies A & B. However B shares were allotted to me recently due to de-merger between A & B.

'A' company's shares had been there for over 2 years. However after de-merger A's price got corrected from say 200 Rs to 100 Rs.

However effectively my total stock valuation (A + B) was still same because B's proportionate shares got allotted to me may be as per their company decision.

How do I handle this situation in tax computation? Will gains from A & B be still calculated based on their 31-Jan-18 price?  'A' will then appear as significant loss.

Any clarity on this situation would be great to know.

Thanks and regards. Rajesh