Basic and diluted net loss per common share is presented in conformity with the two-class method required for participating securities. Holders of Series F preferred stock were entitled to receive cumulative dividends at the annual rate of 10% compounded quarterly payable prior and in preference to any dividends on any shares of our common stock, subject to certain adjustments as set forth in our certificate of incorporation. In the event a dividend is paid on common stock, the holders of preferred stock were entitled to a proportionate share of any such dividend as if they were holders of common stock (on an as-if converted basis). Accordingly, all of our outstanding series of preferred stock were considered to be participating securities. The holders of our preferred stock did not have a contractual obligation to share in our losses; therefore, no amount of total undistributed loss was allocated to preferred stock. Net loss attributable to common stockholders was calculated as net loss less current period preferred stock dividends. There was no preferred stock outstanding during the year ended December 31, 2021.

Basic net loss per share attributable to common stockholders is computed by dividing net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, which includes both Series 1 and Series 2 outstanding shares. Because we have reported a net loss for the years ended December 31, 2021, 2020, and 2019, the number of shares used to calculate diluted net loss per share of common stock attributable to common stockholders is the same as the number of shares used to calculate basic net loss per share of common stock attributable to common stockholders for the period presented because the potentially dilutive shares would have been antidilutive if included in the calculation. Series 1 and Series 2 have the same rights and privileges except Series 2 are not entitled to vote on any matter except as required by law. A pre-IPO preferred shareholder received Series 2 upon the conversion of their preferred shares at the time of our initial public offering. These Series 2 automatically convert to Series 1 upon a qualifying disposition of the shares by the shareholder. A total of 5.1 million shares converted from Series 2 to Series 1 during the year ended December 31, 2021. There are no Series 2 shares outstanding as of December 31, 2021.

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted-average shares outstanding because such securities would have been antidilutive:

	Year ended December 31,
(in thousands)	2021		2020		2019
Preferred stock as-converted		—		—		34,442
Stock options outstanding		5,684		8,215		9,327
Acquisition related compensation ⁽¹⁾		1,756
Warrants to purchase common stock		—		—		364
Restricted stock units		2,331		1,408		—
Convertible debt		4,719		—		2,180
Total potentially dilutive securities		14,490		9,623		46,313