How to calculate tax on ESPP ( Employee stock purchase plan?

Income Tax Income from Capital Gains
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Hi,

My company give me option to buy company (US Listed) with ESPP, where I gets shared at discounted rate and I can buy with max 15% of monthly salary.

Assume that share price was 100 but I got 10% discounted price 90.

This 10% get added to my income in ESPP perquisite and get charged at slab rate.

What could be cost of acquisition for me when I sell those shares? Is it 90 or 100?

Stock exchange ledger will show that bought at 90 but I also paid tax on 10 so will it be 100 or still 90 and gain will be calculated with sellPrice - 90?

Thanks