What was the change between the beginning of period balance and end of period balance in 2019?
{"Beginning of period balance": {"2019": 2881, "2018": "$\u2014"}, "Commission costs and upfront payments to a customer capitalized in period": {"2019": 4141, "2018": 4864}, "Amortization of contract assets": {"2019": -2444, "2018": -1983}, "End of period balance": {"2019": 4578, "2018": 2881}}
end_of_period_balance_2019 = df["End of period balance"]["2019"]
beginning_of_period_balance_2019 = df["Beginning of period balance"]["2019"]
answer = end_of_period_balance_2019 - beginning_of_period_balance_2019