You enter the actual amount of *your* own pre 2018 contributions that you have not already remove with other distributions . The increase in value (or loss) is not included. Contributions are always distributed before earnings from a Roth.
Note: if your age is 59 1/2 or more, then the distribution is not subject to any early distribution penalty anyway so you can just enter zero and not worry about the amount since it will make no difference.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**