What was the percentage change in the Balance at end of the period between 2018 and 2019?
{"Balance at beginning of period": {"2019": 11274, "2018": 17067, "2017": 18176}, "Remediation expense": {"2019": 2602, "2018": 1182, "2017": 307}, "Remediation payments": {"2019": -2455, "2018": -6967, "2017": -1416}, "Other activity (1)": {"2019": 23, "2018": -8, "2017": "\u2014"}, "Balance at end of the period": {"2019": 11444, "2018": 11274, "2017": 17067}}
balance_2019 = df["Balance at end of the period"]["2019"]
balance_2018 = df["Balance at beginning of period"]["2019"]
answer = (balance_2019 - balance_2018) / balance_2018 * 100.0