You track the contribution as basis.

https://www.irs.gov/taxtopics/tc451

A Roth IRA differs from a traditional IRA in several ways. Contributions to a Roth IRA aren't deductible (and you don't report the contributions on your tax return), but qualified distributions or distributions that are a return of contributions aren't subject to tax. To be a Roth IRA, the account or annuity must be designated as a Roth IRA when it's set up. For more information on Roth IRA contributions, refer to Topic No. 309 and to determine if a distribution from your Roth IRA is taxable, see Is the Distribution From My Roth Account Taxable?