What was the change in the balance at beginning of period between 2018 and 2019?
{"Balance at beginning of period": {"April 26, 2019": 3363, "April 27, 2018": 3213}, "Additions": {"April 26, 2019": 2763, "April 27, 2018": 2566}, "Revenue recognized during the period": {"April 26, 2019": -2458, "April 27, 2018": -2416}, "Balance at end of period": {"April 26, 2019": 3668, "April 27, 2018": 3363}}
balance_2019 = df["Balance at beginning of period"]["April 26, 2019"]
balance_2018 = df["Balance at beginning of period"]["April 27, 2018"]
answer = balance_2019 - balance_2018