17. STOCK-BASED COMPENSATION

Under our 2015 Stock Incentive Plan (the "2015 Plan"), 6,000,000 shares of Class A common stock are authorized for grants of stock options, restricted stock units ("RSU"), restricted stock and stock appreciation rights to employees and non-employee directors. In addition, any shares available under our Amended and Restated Stock Incentive Plan (which expired on June 1, 2015), including in respect of any awards that expire, terminate, are forfeited or withheld, will be available for awards under the 2015 Plan. Under the 2015 Plan, awards are made to employees and directors at the discretion of the Compensation Committee. Any awards previously issued under the Amended and Restated Stock Incentive Plan will continue to be governed by the terms of that plan.

For the years ended December 31, 2018, 2017 and 2016, we recognized charges for stock-based compensation of US$ 7.1 million, US$ 4.4 million and US$ 3.5 million, respectively, presented as a component of selling, general and administrative expenses in our consolidated statements of operations and comprehensive income / loss.

Stock Options

Grants of options allow the holders to purchase shares of Class A common stock at an exercise price, which is generally the market price prevailing at the date of the grant, with vesting between one and four years after the awards are granted. There was no option activity during the year ended December 31, 2018. The summary of stock options outstanding as at December 31, 2018 and December 31, 2017 is presented below:



	Shares	Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value
Outstanding at December 31, 2017	2,011,392	$	2.32	7.58	$	4,677
Outstanding at December 31, 2018	2,011,392	$	2.32	6.58	$	916
Vested or expected to vest at December 31, 2018	2,011,392	$	2.32	6.58	$	916
Exercisable at December 31, 2018	1,405,696	$	2.31	6.53	$	654

When options are vested, holders may exercise them at any time up to the maximum contractual life of the instrument which is specified in the option agreement. At December 31, 2018, the maximum life of options that were issued under the 2015 Plan was ten years. Upon providing the appropriate written notification, holders pay the exercise price and receive shares.

The aggregate intrinsic value (the difference between the stock price on the last day of trading of the fourth quarter of 2018 and the exercise prices multiplied by the number of in-the-money options) represents the total intrinsic value that would have been received by the option holders had they exercised all in-the-money options as at December 31, 2018. This amount changes based on the fair value of our Class A common stock. As at December 31, 2018, there was US$ 0.4 million of unrecognized compensation expense related to stock options which is expected to be recognized over a weighted-average period of 0.7 years.

The following table summarizes information about stock option activity during 2018, 2017, and 2016:



	2018			2017			2016
	Shares	Weighted Average Exercise Price (per share)		Shares	Weighted Average Exercise Price (per share)		Shares		Weighted Average Exercise Price (per share)
Outstanding at January 1	2,011,392	$	2.32	2,011,392	$	2.32	1,666,000		$	3.53
Awards granted	—	—		—	—		411,392		2.46
Awards expired	—	—		—	—		(66,000	)	33.66
Outstanding at December 31	2,011,392	$	2.32	2,011,392	$	2.32	2,011,392		$	2.32

Restricted Stock Units

Each RSU represents a right to receive one share of Class A common stock of the Company for each RSU that vests in accordance with a time-based vesting schedule, generally between one to four years from the date of grant. Holders of RSU awards are not entitled to receive cash dividend equivalents and are not entitled to vote.

The following table summarizes information about unvested RSUs as at December 31, 2018:



	Number of Shares / Units		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2017	1,974,954		$	3.21
Granted	962,369		4.28
Vested	(940,968	)	3.31
Unvested at December 31, 2018	1,996,355		$	3.68

The intrinsic value of unvested RSUs was US$ 5.5 million as at December 31, 2018. Total unrecognized compensation cost related to unvested RSUs as at December 31, 2018 was US$ 5.2 million and is expected to be recognized over a weighted-average period of 2.5 years.

Restricted Stock Units with Performance Conditions

Each RSU with performance conditions (“PRSU”) represents a right to receive one share of Class A common stock of the Company for each PRSU that vests in accordance with a performance-based vesting schedule, based on meeting specified objectives for unlevered free cash flow and OIBDA over defined periods and by defined dates. Holders of PRSU awards are not entitled to receive cash dividend equivalents and are not entitled to vote.

Under the 2015 PRSU Award, 239,703 PRSUs vested in March 2018. The completion of the sale of the Company's Croatian operations in July resulted in the vesting of the remaining unvested 2015 PRSUs together with an additional 461,188 PRSUs that were granted and immediately vested in accordance with the terms of the 2015 PRSU Award agreement.

On December 4, 2018, the 2018 PRSU Award was granted with unlevered free cash flow and OIBDA targets corresponding to two, three and four-year performance periods ended December 31, 2020, 2021 and 2022, respectively. The maximum achievement under the 2018 PRSU Award is 200% of the shares allotted to the corresponding target. Due to the uncertainty of achieving any of the prescribed targets within the 2018 PRSU Award, we have not recognized any related compensation cost.

The following table summarizes information about unvested PRSUs as at December 31, 2018:



	Number of Shares / Units		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2017	719,109		$	2.67
Granted	962,760		3.52
Vested	(1,180,297	)	3.14
Unvested at December 31, 2018	501,572		$	3.19

The intrinsic value of unvested PRSUs was US$ 1.4 million as at December 31, 2018. Total unrecognized compensation cost related to unvested PRSUs as at December 31, 2018 was US$ 1.6 million.