What is the change in the Net favorable impact to income before income taxes from 2018 to 2017?
{"": {"January 3,2020": "", "December 28,2018": "(in millions, except for per share amounts)", "December 29, 2017": ""}, "Favorable impact": {"January 3,2020": 95, "December 28,2018": 167, "December 29, 2017": 185}, "Unfavorable impact": {"January 3,2020": -52, "December 28,2018": -62, "December 29, 2017": -82}, "Net favorable impact to income before income taxes": {"January 3,2020": 43, "December 28,2018": 105, "December 29, 2017": 103}, "Impact on diluted EPS attributable to Leidos common stockholders": {"January 3,2020": 0.23, "December 28,2018": 0.52, "December 29, 2017": 0.41}}
net_favorable_impact_to_income_2018 = df["Net favorable impact to income before income taxes"]["December 28,2018"]
net_favorable_impact_to_income_2017 = df["Net favorable impact to income before income taxes"]["December 29, 2017"]
answer = net_favorable_impact_to_income_2018 - net_favorable_impact_to_income_2017