Under the evergreen provision in our 2018 Equity Incentive Plan ("2018 Plan"), we reserved an additional 4,929,361 shares of our common stock for issuance in January 2020. At March 31, 2020, there were 17,555,416 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 March 31,
(in thousands)	2020		2019
Cost of revenue	$	747	$	652
Sales and marketing	4,496		3,366
Research and development	2,948		2,030
General and administrative	4,844		3,271
Total stock-based compensation expense	$	13,035	$	9,319

At March 31, 2020, the unrecognized stock-based compensation expense related to unvested restricted stock units was $134.5 million, which is expected to be recognized over an estimated remaining weighted average period of 3.4 years.

At March 31, 2020, the total unrecognized stock-based compensation expense related to outstanding stock options was $20.3 million, which is expected to be recognized over an estimated remaining weighted average period of 1.9 years.

At March 31, 2020, the unrecognized stock-based compensation expense related to unvested awards of restricted stock was $1.3 million, which is expected to be recognized over an estimated remaining period of 0.8 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	—		—		3,082		28.00
Vested	(99	)	4.25		(403	)	29.14
Forfeited	—		—		(123	)	26.16
Unvested balance at March 31, 2020	396		4.25		5,450		27.07



(in thousands, except for per share data 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	(635)	6.27			12,383
Forfeited/canceled	(190)	10.78
Outstanding at March 31, 2020	12,114	8.45		7.0	162,596
Exercisable at March 31, 2020	6,783	6.12		6.5	106,847

At March 31, 2020, there were 12.1 million stock options that were vested and expected to vest.

Under the evergreen provision in our 2018 Employee Stock Purchase Plan ("2018 ESPP"), we reserved an additional 1,478,808 shares of our common stock for issuance in January 2020. At March 31, 2020, there were 5,721,517 shares reserved for issuance under the 2018 ESPP.

In the three months ended March 31, 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 March 31, 2020, there was $1.7 million of employee contributions to the 2018 ESPP included in accrued compensation. The unrecognized stock-based compensation expense related to our 2018 ESPP was $8.7 million, which is expected to be recognized over the remaining weighted average period of 0.8 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:



	Three Months Ended March 31,
	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	—	—