Prospectus excerpt:  We are offering 2,750,000 shares of 6.50% Series B Cumulative Convertible Perpetual Preferred Stock (the "Series B convertible preferred stock"). The annual dividend on each share of Series B convertible preferred stock is $6.50 and is payable, when, as and if declared by our board of directors, quarterly in cash, in arrears, on each January 15, April 15, July 15 and October 15, commencing on October 15, 2018. Each share of Series B convertible preferred stock has a liquidation preference of $100.00 per share and is convertible, at the holder's option at any time, initially into 2.1264 shares of our Class A common stock (equal to an initial conversion price of approximately $47.03 per share), subject in each case to specified adjustments as set forth in this prospectus supplement. If a fundamental change occurs, we may be required to pay a make-whole premium on Series B convertible preferred stock converted in connection therewith, as described in this prospectus supplement. The Series B convertible preferred stock is not redeemable by us. On or after July 20, 2023, we may, at our option, cause all outstanding shares of the Series B convertible preferred stock to be automatically converted into that number of shares of Class A common stock for each share of Series B convertible preferred stock equal to the then-prevailing conversion rate if the closing sale price of our Class A common stock equals or exceeds 150% of the then-prevailing conversion price for at least 20 trading days in a period of 30 consecutive trading days, including the last trading day of such 30-day period.