Enter the Form 1099-R exactly as received under Wages & Income -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plan Withdrawals (1099-R) and answer the immediate follow-up questions. After that, click the Continue button on the Your 1099-R Entries page. When TurboTax asks for it, enter you basis in Roth IRA contributions for years prior to 2017.
If you made any contribution for 2017, be sure to enter that under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions and answer the follow-up questions.
TurboTax will use this information to prepare Form 8606 Part III to calculate the taxable amount of the distribution. If all of the distribution came from your original contributions, the result will be that none of the distribution is taxable or subject to early-distribution penalty.
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