A summary of shares repurchased under our stock repurchase program authorized by our Board of Directors follows:


	2020		2019		2018
Shares repurchased	7.2		1.3		2.1
Aggregate purchase price	$	382.3	$	44.7	$	100.0
Average purchase price per share	$	52.76	$	35.51	$	47.58

As of December 31, 2020, $197.8 million remained available under our stock repurchase limit most recently authorized by our Board of Directors. In February 2021, our Board of Directors increased the share repurchase authorization by $1 billion.

We have 5.0 million authorized shares of preferred stock, par value $0.01 per share, none of which are issued or outstanding. The Board of Directors has the authority to issue the preferred stock in one or more series and to establish the rights, preferences, and dividends.

A summary of shares of common stock issued in connection with the exercise of stock options follows:


	2020		2019		2018
Shares issued	1.1		0.4		0.5
Proceeds from the exercise of stock options	$	52.7	$	12.7	$	17.8
Average exercise price per share	$	49.91	$	33.87	$	35.25

The following table presents a summary of shares of common stock issued in connection with the settlement of RSUs, as well as shares surrendered to AutoNation to satisfy tax withholding obligations in connection with the vesting of restricted stock and settlement of RSUs:


	2020	2019	2018
Shares issued	0.5	0.3	0.1
Shares surrendered to AutoNation to satisfy tax withholding obligations	0.2	0.1	0.1