What is the difference in the number of company-operated restaurants between 2017 and 2018 at end of period total? 
{"Company-operated restaurants: -- Beginning of period": {"2019": 137, "2018": 276, "2017": 417, "2016": 413, "2015": 431}, "Company-operated restaurants: -- New": {"2019": "\u2014", "2018": 1, "2017": 2, "2016": 4, "2015": 2}, "Company-operated restaurants: -- Refranchised": {"2019": "\u2014", "2018": -135, "2017": -178, "2016": -1, "2015": -21}, "Company-operated restaurants: -- Closed": {"2019": "\u2014", "2018": -5, "2017": -15, "2016": "\u2014", "2015": -6}, "Company-operated restaurants: -- Acquired from franchisees": {"2019": "\u2014", "2018": "\u2014", "2017": 50, "2016": 1, "2015": 7}, "Company-operated restaurants: -- End of period total": {"2019": 137, "2018": 137, "2017": 276, "2016": 417, "2015": 413}, "Company-operated restaurants: -- % of system": {"2019": 6, "2018": 6, "2017": 12, "2016": 18, "2015": 18}, "Franchise restaurants: -- Beginning of period": {"2019": 2100, "2018": 1975, "2017": 1838, "2016": 1836, "2015": 1819}, "Franchise restaurants: -- New": {"2019": 19, "2018": 11, "2017": 18, "2016": 12, "2015": 16}, "Franchise restaurants: -- Refranchised": {"2019": "\u2014", "2018": 135, "2017": 178, "2016": 1, "2015": 21}, "Franchise restaurants: -- Closed": {"2019": -13, "2018": -21, "2017": -9, "2016": -10, "2015": -13}, "Franchise restaurants: -- Sold to company": {"2019": "\u2014", "2018": "\u2014", "2017": -50, "2016": -1, "2015": -7}, "Franchise restaurants: -- End of period total": {"2019": 2106, "2018": 2100, "2017": 1975, "2016": 1838, "2015": 1836}, "Franchise restaurants: -- % of system": {"2019": 94, "2018": 94, "2017": 88, "2016": 82, "2015": 82}, "Franchise restaurants: -- System end of period total": {"2019": 2243, "2018": 2237, "2017": 2251, "2016": 2255, "2015": 2249}}
company_restaurants_2018 = df["Company-operated restaurants: -- Beginning of period"]["2018"]
company_restaurants_2019 = df["Company-operated restaurants: -- Beginning of period"]["2019"]
answer = company_restaurants_2018 - company_restaurants_2019