What was the change in contribution income from 2017 to 2018?
{"": {"2019": "", "2018": "(in thousands, except percentage data)", "2017": ""}, "Net revenue": {"2019": 287372, "2018": 287756, "2017": 270908}, "Percentage of net revenue": {"2019": 28.8, "2018": 27.2, "2017": 26.1}, "Contribution income": {"2019": 67282, "2018": 70142, "2017": 63865}, "Contribution margin": {"2019": 23.4, "2018": 24.4, "2017": 23.6}}
contribution_income_2018 = df["Contribution income"]["2018"]
contribution_income_2017 = df["Contribution income"]["2017"]
answer = contribution_income_2018 - contribution_income_2017