What is the difference in nonvested stock awards and units between 2018 and 2019?
{"Weighted-average shares outstanding  basic": {"2019": 25823, "2018": 28499, "2017": 30630}, "Effect of potentially dilutive securities: -- Nonvested stock awards and units": {"2019": 211, "2018": 240, "2017": 182}, "Effect of potentially dilutive securities: -- Stock options": {"2019": 10, "2018": 40, "2017": 59}, "Effect of potentially dilutive securities: -- Performance share awards": {"2019": 24, "2018": 28, "2017": 43}, "Effect of potentially dilutive securities: -- Weighted-average shares outstanding  diluted": {"2019": 26068, "2018": 28807, "2017": 30914}, "Excluded from diluted weighted-average shares outstanding: -- Antidilutive": {"2019": 186, "2018": 150, "2017": 76}, "Excluded from diluted weighted-average shares outstanding: -- Performance conditions not satisfied at the end of the period": {"2019": 65, "2018": 44, "2017": 53}}
nonvested_2018 = df["Effect of potentially dilutive securities: -- Nonvested stock awards and units"]["2018"]
nonvested_2019 = df["Effect of potentially dilutive securities: -- Nonvested stock awards and units"]["2019"]
answer = nonvested_2018 - nonvested_2019