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Stock Option Plans (Tables) | 6 Months Ended
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Jun. 30, 2020
Stock Option Plans |
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Options Outstanding
​ |  | ​ |  | ​ |  | Weighted ‑
​ | ​ | Shares | ​ | ​ | ​ | Average
​ | ​ | Available | ​ | Number of | ​ | Exercise
​ |  | for Grant |  | Options |  | Price
Balance at December 31, 2019 |  | 1,704,183 |  | 5,643,410 | ​ | $ | 8.05
Additional shares authorized |  | 1,118,000 |  | — | ​ |  | —
Options granted |  | (190,000) |  | 190,000 | ​ |  | 2.68
Options exercised |  | — |  | (81,500) | ​ |  | 0.28
Options forfeited |  | 190,812 |  | (190,812) | ​ |  | 7.07
Balance at June 30, 2020 |  | 2,822,995 |  | 5,561,098 | ​ | $ | 8.01

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