What was the percentage change in balance of restricted stock units outstanding from 2018 to 2019?
{"Balance of restricted stock units outstanding at December 31, 2018": {"Number of Shares": 505000.0, "Weighted-Average Grant Date Fair Value": 2.17}, "Grants of restricted stock units": {"Number of Shares": 70000.0, "Weighted-Average Grant Date Fair Value": 2.45}, "Vested restricted stock units": {"Number of Shares": -235000.0, "Weighted-Average Grant Date Fair Value": -2.29}, "Balance of unvested restricted stock units at December 31, 2019": {"Number of Shares": 340000.0, "Weighted-Average Grant Date Fair Value": 2.15}}
balance_2019 = df["Balance of unvested restricted stock units at December 31, 2019"]["Number of Shares"]
balance_2018 = df["Balance of restricted stock units outstanding at December 31, 2018"]["Number of Shares"]
answer = ( balance_2019 - balance_2018 ) / balance_2018 * 100.0