We granted restricted stock units (“RSUs”) in July 2019 to certain members of management, pursuant to retention agreements entered into with these employees (the “IPO Grants”). These RSUs vest upon satisfaction of both a performance-based vesting condition, which was satisfied when we completed our IPO on February 4, 2020, and a service-based vesting condition, which will be satisfied with respect to one-third of an employee’s RSUs on each anniversary from the date of our IPO for three consecutive years, subject to the employee’s continued employment through the applicable vesting date.

In addition, in conjunction with our Corporate Reorganization and IPO in February 2020, we established an equity incentive plan for purposes of granting stock-based compensation awards to certain of our senior management, our non-executive directors and to certain employees, to incentivize their performance and align their interests with ours. We have granted RSUs to certain employees and non-employee directors that have a service-based vesting condition. In addition, we have granted performance stock units (“PSUs”) to certain members of management that have a performance-based vesting condition. We account for forfeitures of outstanding but unvested grants in the period they occur. A maximum of 10.5 million shares of common stock were initially available for issuance under equity incentive awards granted pursuant to the plan. In the years ended December 31, 2021 and 2020, 0.2 million and 0.3 million RSUs and 0.2 million and 0.2 million PSUs were granted, respectively.

A summary of activity for RSUs and PSUs for the years ended December 31, 2021 and 2020, is as follows (in millions, except for per share data):

	Shares			Weighted-Average Grant-Date Fair Value Per Share
Unvested, at January 1, 2020		—		$	—
Granted	0.5				29
Forfeited		(0.1	)		27
Vested		—			—
Unvested, at December 31, 2020		0.4		$	29
Granted		0.3			30
Forfeited		—			—
Vested		(0.1	)		28
PSU performance adjustment		(0.2	)		30
Unvested, at December 31, 2021		0.4		$	29

Unrecognized compensation expense relating to unvested RSUs and PSUs as of December 31, 2021, was $6 million, which is expected to be recognized over a weighted average period of 1.4 years.

There were stock-based compensation awards representing 0.4 million shares outstanding at December 31, 2021 and 2020. Stock-based compensation expense was $4 million and $5 million for the years ended December 31, 2021 and 2020, respectively.