What was the change in the Net income attributable to common stockholders between 2018 and 2019?
{"Revenue": {"2019": 527550.0, "2018": 451013.0}, "Net income attributable to common stockholders": {"2019": 51075.0, "2018": 13264.0}, "Net income attributable to common stockholders per share - basic": {"2019": 1.05, "2018": 0.27}, "Net income attributable to common stockholders per share - diluted": {"2019": 1.02, "2018": 0.26}}
net_income_2019 = df["Net income attributable to common stockholders"]["2019"]
net_income_2018 = df["Net income attributable to common stockholders"]["2018"]
answer = net_income_2019 - net_income_2018