Under the evergreen provision in our 2018 Equity Incentive Plan ("2018 Plan"), in January 2020 we reserved an additional 4,929,361 shares of our common stock for issuance. At June 30, 2020, there were 17,500,941 shares available for grant under the 2018 Plan.

Stock-based compensation expense included in the consolidated statements of operations was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2020		2019		2020		2019
Cost of revenue	$	830	$	742	$	1,577	$	1,394
Sales and marketing	5,375		4,215		9,871		7,581
Research and development	3,893		2,441		6,841		4,471
General and administrative	5,568		3,975		10,412		7,246
Total stock-based compensation expense	$	15,666	$	11,373	$	28,701	$	20,692

At June 30, 2020, the unrecognized stock-based compensation expense related to unvested restricted stock units was $127.9 million, which is expected to be recognized over an estimated remaining weighted average period of 3.2 years.

At June 30, 2020, the unrecognized stock-based compensation expense related to outstanding stock options was $17.0 million, which is expected to be recognized over an estimated remaining weighted average period of 1.7 years.

At June 30, 2020, the unrecognized stock-based compensation expense related to unvested awards of restricted stock was $0.8 million, which is expected to be recognized over an estimated remaining period of 0.5 years.

A summary of our restricted stock and restricted stock units activity is presented below:



	Restricted Stock				Restricted Stock Units
(in thousands, except for per share data)	Number of Shares		Weighted Average Grant Date Fair Value		Number of Shares		Weighted Average Grant Date Fair Value
Unvested balance at December 31, 2019	495		$	4.25	2,894		$	26.34
Granted	—		—		3,372		27.86
Vested	(198	)	4.25		(569	)	29.04
Forfeited	—		—		(261	)	25.71
Unvested balance at June 30, 2020	297		4.25		5,436		27.03



(in thousands, except for exercise prices and years)	Number of Shares	Weighted Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at December 31, 2019	12,939	$	8.38	7.1	$	201,608
Granted	—	—
Exercised	(1,594)	6.89			33,416
Forfeited/canceled	(287)	10.99
Outstanding at June 30, 2020	11,058	8.53		6.8	235,370
Exercisable at June 30, 2020	6,692	6.42		6.4	156,517

At June 30, 2020, there were 11.1 million stock options that were vested and expected to vest.

Under the evergreen provision in our 2018 Employee Stock Purchase Plan ("2018 ESPP"), in January 2020 we reserved an additional 1,478,808 shares of our common stock for issuance. At June 30, 2020, there were 5,721,517 shares reserved for issuance under the 2018 ESPP.

In the six months ended June 30, 2020, employees purchased 377,378 shares of our common stock at a weighted average price of $19.36 per share, resulting in $7.3 million of cash proceeds.

At June 30, 2020, there was $5.0 million of employee contributions to the 2018 ESPP included in accrued compensation. The unrecognized stock-based compensation expense related to our 2018 ESPP was $6.5 million, which is expected to be recognized over the remaining weighted average period of 0.7 years.

The fair value of the 2018 ESPP purchase rights was estimated on the offering or modification dates using a Black-Scholes option-pricing model and the following assumptions:



	Six Months Ended June 30,
	2020	2019
Expected term (in years)	0.5 — 2.0	0.5 — 2.0
Expected volatility	41.6% — 47.9%	35.4% — 44.6%
Risk-free interest rate	0.8% — 0.9%	2.5% — 2.6%
Expected dividend yield	—	—