By how much less did the company spend for stock-based compensation in 2019 compared to 2018?
{"Year Ended May 31, -- ": {"2019": "", "Actual": "", "Constant": "Percent Change", "2018": ""}, "Year Ended May 31, -- (Dollars in millions)": {"2019": 2019, "Actual": "Actual", "Constant": "Constant", "2018": 2018}, "Year Ended May 31, -- General and administrative (1)": {"2019": 1093, "Actual": -1, "Constant": 2, "2018": 1102}, "Year Ended May 31, -- Stock-based compensation": {"2019": 172, "Actual": -5, "Constant": -5, "2018": 180}, "Year Ended May 31, -- Total expenses": {"2019": 1265, "Actual": -1, "Constant": 1, "2018": 1282}, "Year Ended May 31, -- % of Total Revenues": {"2019": 3, "Actual": "", "Constant": "", "2018": 3}}
stock_2018 = df["Year Ended May 31, -- Stock-based compensation"]["2018"]
stock_2019 = df["Year Ended May 31, -- Stock-based compensation"]["2019"]
answer = stock_2018 - stock_2019