Restricted Stock and Restricted Stock Units
Historically, we have granted shares of restricted stock to certain employees that have vested in three equal annual installments on the anniversary dates of their grant. However, beginning in 2019, we altered our approach for these grants to replace the grant of restricted stock subject to time-based vesting with the grant of a combination of restricted stock units ("RSUs") subject to time- based vesting and performance-based vesting. Each RSU represents the contingent right to receive a single share of our common stock upon the vesting of the award. For the year ended December 31, 2019, we granted an aggregate of 280,000 RSUs to certain employees. Of the RSUs granted during 2019, 217,000 of such RSUs are subject to time-based vesting and are scheduled to vest in three equal annual installments on the anniversary dates of the grant. The remaining 63,000 RSUs are performance-based awards that will vest based on our achievement of long-term performance goals, in particular, based on our levels of 2021 revenue and operating income. The 63,000 shares issuable upon vesting of the performance-based RSUs represent the maximum payout under our performance-based awards, based upon 150% of our target performance for 2021 revenue and operating income (the payout of such awards based on target performance for 2021 revenue and operating income would be 42,000 shares). In the case of the time-based and performance-based RSUs, vesting is also subject to the employee's continuous service with us through vesting. In 2018, we granted 280,000 shares of restricted stock to certain employees. Shares issued to employees vest in three equal annual installments on the anniversary dates of their grant. In 2019 and 2018, 194,333 and 182,500 shares of restricted stock vested, respectively.
In addition, in conjunction with our 2018 and 2019 Annual Meetings of Stockholders, we granted RSUs to certain members of our Board of Directors in respect of the annual equity compensation under our non-employee director compensation policy (other members of our Board of Directors elected to receive their annual equity compensation for Board service in the form of stock units under our Deferred Compensation Plan as described below). In 2019 and 2018, we granted 11,600 and 16,286, respectively, RSUs to members of our Board of Directors in respect of the annual equity compensation under our non-employee director compensation policy. RSUs issued to our Board of Directors vest at the earlier of the one-year anniversary of their grant or the next annual stockholders' meeting. In 2019 and 2018, 16,286 and 129,865 RSUs, respectively, vested.
The following table summarizes our aggregate restricted stock awards and RSU activity in 2019 and 2018:
We recognized $1.5 million and $0.6 million in stock-based compensation expense, which is recorded in selling, general and administrative expense on the consolidated statement of operations for the years ended December 31, 2019 and 2018, respectively, and we will recognize $4.0 million over the remaining requisite service period for unamortized restricted stock, RSUs and stock options.
Unamortized restricted stock and RSUs expense at December 31, 2019 that will be amortized over the weighted-average remaining service period of 2 years totaled $1.2 million.
Table:
 | Number of Unvested Shares  | Weighted Average Grant Date Fair Value | Aggregate Grant Date Fair Value of Unvested Shares
Balance at January 1, 2017 | 489,698 | $1.51 | 738,345
Granted | 296,287 | $3.07 | $909,600
Vested | (312,365) | $1.45 | $(454,339)
Forfeitures | (15,000) | $1.41 | $(21,150)
Balance at December 31, 2018 | 458,620 | $2.56 | $1,172,456
Granted | 291,600 | $3.75 | $1,094,430
Vested | (210,619) | $2.33 | $(490,769)
Forfeitures | (37,499) | $2.96 | $(111,115)
Balance at December 31, 2019 | 502,102 | $3.32 | $1,665,002


Question: What is the change in Number of Unvested Shares from Balance at January 1, 2017 to December 31, 2018?
Answer: -31078
Calculation: 458,620-489,698