simple depends on what has transpired in the corp since it was formed in 2015. loans and losses can make it complicated
here's a general link.
https://www.bing.com/images/search?q=s-corp+basis+worksheet&qpvt=s-corp+basis+worksheet&form=IGRE&fi...
note that the IRS, in some cases, requires that form 7203 which tracks basis be included with your 1040
https://www.irs.gov/site-index-search?search=7203&field_pup_historical_1=1&field_pup_historical=1
