What was the increase / (decrease) in the Balance at April 1 from 2018 to 2019?
{"Balance at April 1": {"2019": 687, "2018": 988, "2017": 1617}, "Reductions: -- Relating to positions taken during prior year": {"2019": "\u2014", "2018": -300, "2017": -604}, "Reductions: -- Relating to lapse in statute": {"2019": -107, "2018": -1, "2017": -25}, "Reductions: -- Balance at March 31": {"2019": 580, "2018": 687, "2017": 988}}
balance_2019 = df["Balance at April 1"]["2019"]
balance_2018 = df["Balance at April 1"]["2018"]
answer = balance_2019 - balance_2018