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


	Three Months Ended
	March 31,
	2021		2020
Shares repurchased	3.8		2.5
Aggregate purchase price	$	306.1	$	80.0
Average purchase price per share	$	79.76	$	31.95

As of March 31, 2021, $891.7 million remained available for share repurchases under the program.

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 of such preferred stock.

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


	Three Months Ended
	March 31,
	2021		2020
Shares issued	0.4		0.1
Proceeds from the exercise of stock options	$	21.2	$	1.0
Average exercise price per share	$	50.57	$	18.12

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:


	Three Months Ended
	March 31,
	2021	2020
Shares issued	0.6	0.5
Shares surrendered to AutoNation to satisfy tax withholding obligations	0.2	0.2