2/24/2018 S-1 Table of Contents due to headcount growth, and other general and administrative expenses increased $11.3 million related to the funding of the Dropbox Charitable Foundation. Included in this amount was an equity­based charitable contribution of $9.4 million and cash contributions of $1.9 million. Further, general and administrative expenses increased $4.3 million due to legal­related expenses and accounting services. Interest expense, net Interest expense, net decreased $5.4 million during 2017, as compared to 2016, primarily due to a decrease in interest expense of $3.3 million due to fewer assets acquired under capital leases. In addition, interest income from our money market funds increased by $2.1 million. Other income (expense), net Other income (expense), net increased $8.3 million during 2017, as compared to 2016, primarily due to an increase of $8.6 million in foreign currency gains related to monetary assets and liabilities denominated in euros and British pound sterling. Provision for income taxes Provision for income taxes decreased by $5.0 million during 2017 as compared to 2016, primarily as a result of a change in our U.S. AMT position. We were subject to U.S. AMT during 2016, and we do not expect to be subject to U.S. AMT during 2017. In addition, as a result of the repeal of AMT as part of the Tax Reform Act, we recognized a benefit and established a receivable in 2017 to reflect anticipated refunds of our 2016 AMT credit carryforward. Comparison of the year ended December 31, 2015 and 2016 Revenue Year ended December 31, 2015 2016 $ Change % Change (In millions) Revenue $603.8 $844.8 $ 241.0 40% Revenue increased $241.0 million or 40% during 2016 as compared to 2015. This increase was primarily due to a 35% increase in the number of paying users between periods. The average revenue per paying user also decreased slightly between periods. Cost of revenue, gross profit, and gross margin Year ended December 31, 2015 2016 $ Change % Change (In millions) Cost of revenue $407.4 $390.6 $ (16.8) (4)% Gross profit 196.4 454.2 257.8 131% Gross margin 33% 54% Cost of revenue decreased $16.8 million or 4% during 2016, as compared to 2015, primarily due to a net decrease of $39.5 million in our infrastructure costs due to our Infrastructure Optimization. The net decrease of $39.5 million included a $92.5 million decrease in expense related to our third­party datacenter service provider, offset by a $53.0 million increase in depreciation, facilities, and support expense related to our infrastructure 73 https://www.sec.gov/Archives/edgar/data/1467623/000119312518055809/d451946ds1.htm 81/235