The dilutive potential shares common stock is summarized as follows:


	At March 31,
	2020	2019
Stock options outstanding (1)	19,803,872	21,109,998
Restricted stock outstanding (2)	4,600,227	2,372,347
Common stock warrants (3)	110,573,392	115,824,142
Preferred stock (4)	2,782,075	17,933,591
Convertible Senior Notes (5)	59,133,896	43,630,020
Number of dilutive potential shares of common stock	196,893,462	200,870,098

(1)

During the three months ended March 31, 2020 and 2019, the Company granted zero and 25,000 stock options, respectively.

(2)

During the three months ended March 31, 2020 and 2019, the Company granted zero and 25,000 shares of restricted stock, respectively.

(3)

In April 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement with Amazon, subject to certain vesting events, as described in Note 11, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of March 31, 2020.

In July 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement with Walmart, subject to certain vesting events, as described in Note 11, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of March 31, 2020.

(4)

The preferred stock amount represents the dilutive potential on the shares of common stock as a result of the conversion of the Series C Redeemable Convertible Preferred Stock (Series C Preferred Stock) and Series E Redeemable Preferred Stock (Series E Preferred S)tock, based on the conversion price of each preferred stock as of March 31 2020, and 2019, respectively. Of the 10,431 shares of Series C Preferred Stock issued on May 16, 2013, 7,811 shares had been converted to common stock as of March 31, 2020, with the remainder still outstanding. On November 1, 2018, the Company issued 35,000 shares of Series E redeemable convertible preferred stock (Series E Preferred Stock). As of December 31, 2019, 30,462 shares of the Series E Preferred Stock had been converted to common stock and 4,038 shares were redeemed for cash. The remaining 500 shares of Series E Preferred Stock were converted to common stock in January 2020.

In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes. In September 2019, the Company issued a $40.0 million in aggregate principal amount of 7.5% Convertible Senior Note. See Note 8, Convertible Senior Notes.