What was the percentage change in the net pension period cost between 2018 and 2019?
{"Service costs": {"2019": 1.5, "2018": 2.2, "2017": 1.4}, "Interest costs": {"2019": 1.1, "2018": 1.0, "2017": 1.0}, "Amortization of actuarial loss": {"2019": 0.4, "2018": 0.8, "2017": "\u2014"}, "Settlements": {"2019": "\u2014", "2018": "\u2014", "2017": 0.5}, "Net pension period cost": {"2019": 3.0, "2018": 4.0, "2017": 2.9}}
net_pension_period_cost_2019 = df["Net pension period cost"]["2019"]
net_pension_period_cost_2018 = df["Net pension period cost"]["2018"]
answer = (net_pension_period_cost_2019 - net_pension_period_cost_2018) / net_pension_period_cost_2018 * 100.0