What is the percentage change in the average price between March 31, 2018 and 2019?
{"Outstanding at March 31, 2018": {"Number of Shares": 1894, "Average Price (a)": 12.9, "Average Life (years) (b)": "-", "Aggregate Intrinsic Value": "-"}, "Options granted": {"Number of Shares": "-", "Average Price (a)": "-", "Average Life (years) (b)": "-", "Aggregate Intrinsic Value": "-"}, "Options exercised": {"Number of Shares": -326, "Average Price (a)": 13.11, "Average Life (years) (b)": "-", "Aggregate Intrinsic Value": 1944}, "Options cancelled/forfeited": {"Number of Shares": -122, "Average Price (a)": 13.22, "Average Life (years) (b)": "-", "Aggregate Intrinsic Value": 295}, "Outstanding at March 31, 2019": {"Number of Shares": 1446.0, "Average Price (a)": 12.82, "Average Life (years) (b)": 3.29, "Aggregate Intrinsic Value": 6528.0}, "Exercisable at March 31, 2019": {"Number of Shares": 1347.0, "Average Price (a)": 12.7, "Average Life (years) (b)": 3.08, "Aggregate Intrinsic Value": 6253.0}}
average_price_2019 = df["Exercisable at March 31, 2019"]["Average Price (a)"]
average_price_2018 = df["Outstanding at March 31, 2018"]["Average Price (a)"]
answer = (average_price_2019 - average_price_2018) / average_price_2018 * 100.0