The following table summarizes the unvested stock option activity during the period from January 1, 2015 to December 31, 2017:
| Number of shares | ||||
| Unvested January 1, 2015 | 62,031 | |||
| Granted | 125,500 | |||
| Vested | (92,844 | ) | ||
| Cancelled | – | |||
| Unvested December 31, 2015 | 94,687 | |||
| Granted | 313,000 | |||
| Vested | (266,837 | ) | ||
| Cancelled | (31,250 | ) | ||
| Unvested December 31, 2016 | 109,600 | |||
| Granted | – | |||
| Vested | (99,249 | ) | ||
| Cancelled | (7,500 | ) | ||
| Unvested December 31, 2017 | 2,851 | |||