What was the total change in Additions for tax positions of prior years between 2017 and 2019?
{"Balance at beginning of period": {"April 26, 2019": 348, "April 27, 2018": 218, "April 28, 2017": 216}, "Additions based on tax positions related to the current year": {"April 26, 2019": 11, "April 27, 2018": 131, "April 28, 2017": 7}, "Additions for tax positions of prior years": {"April 26, 2019": 26, "April 27, 2018": "\u2014", "April 28, 2017": 7}, "Decreases for tax positions of prior years": {"April 26, 2019": -35, "April 27, 2018": -1, "April 28, 2017": "\u2014"}, "Settlements": {"April 26, 2019": -54, "April 27, 2018": "\u2014", "April 28, 2017": -12}, "Balance at end of period": {"April 26, 2019": 296, "April 27, 2018": 348, "April 28, 2017": 218}}
additions_2019 = df["Additions for tax positions of prior years"]["April 26, 2019"]
additions_2017 = df["Additions based on tax positions related to the current year"]["April 28, 2017"]
answer = additions_2019 - additions_2017