A summary of the status of stock option activity for the six months ended June 30, 2020 is as follows (in thousands, except price data):


	Shares	Weighted-average exercise price per share
Outstanding at December 31, 2019	944	$	30.63
Granted	122	77.54
Exercised	(220)	21.12
Forfeited	(12)	43.34
Outstanding at June 30, 2020	834	$	39.85

A summary of the status of restricted stock unit activity for the six months ended June 30, 2020 is as follows (in thousands, except price data):


	Shares	Weighted-average grant date fair value
Non-vested December 31, 2019	786	$	41.88
Granted	202	85.34
Vested	(107)	26.07
Forfeited	(18)	58.11
Non-vested at June 30, 2020	863	$	53.71

During the six months ended June 30, 2020, the Company issued 27,826 shares of common stock in connection with the Company’s employee stock purchase plan (the “ESPP”).

During the three months ended June 30, 2020, 247,172 shares of outstanding common stock were repurchased under the Company’s previously announced share repurchase program for approximately $42.2 million. As of June 30, 2020, there was $7.8 million available under the Company’s share repurchase program.

The expense related to the Company’s stock-based compensation plans included in the accompanying Consolidated Statements of Income was as follows (in thousands):


	Three Months Ended June 30,						Six Months Ended June 30,

	2020		2019		2020		2019
Cost of sales	$	435	$	262	$	693	$	542
Research and development	866		544		1,508		1,109
Sales and marketing	1,341		904		2,638		2,023
General and administrative	2,488		1,662		4,169		3,286
Total stock-based compensation expense	$	5,130	$	3,372	$	9,008	$	6,960

As of June 30, 2020, total unrecognized compensation expense was $33.9 million, which is expected to be recognized over a weighted-average period of approximately 2.7 years.

The estimated fair value of each stock option was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions for the option grants.


	Six Months Ended June 30,
	2020			2019
Risk-free interest rate	1.34		%	2.51		%
Expected option life (in years)	5.14			5.68
Volatility rate	39		%	39		%
Dividend rate	0		%	0		%
Weighted-average grant date fair value	$	28.39		$	23.67

The fair value of RSUs is determined based on the closing market price of the Company’s common stock on the grant date. The weighted-average fair value of RSUs granted during the six months ended June 30, 2020 and 2019 was $85.34 and $59.19, respectively.