What is the difference between Benefit obligation at beginning of period for 2018 and 2019?
{"Change in benefit obligations: -- Benefit obligation at beginning of period": {"2019": 46.4, "2018": 51.3}, "Change in benefit obligations: -- Service cost": {"2019": "\u2014", "2018": 0.1}, "Change in benefit obligations: -- Interest cost": {"2019": 1.6, "2018": 1.4}, "Change in benefit obligations: -- Actuarial (gain) loss": {"2019": -1.2, "2018": -1.7}, "Change in benefit obligations: -- Benefits paid, net": {"2019": -3.3, "2018": -4.5}, "Change in benefit obligations: -- Plan amendments": {"2019": "\u2014", "2018": -0.2}, "Change in benefit obligations: -- Benefit obligation at end of period": {"2019": 43.5, "2018": 46.4}, "Change in plan assets: -- Fair value of plan assets at beginning of period": {"2019": "$ \u2014", "2018": "$ \u2014"}, "Change in plan assets: -- Employer contribution": {"2019": 3.3, "2018": 4.5}, "Change in plan assets: -- Benefits paid, net": {"2019": -3.3, "2018": -4.5}, "Change in plan assets: -- Fair value of plan assets at end of period": {"2019": "$ \u2014", "2018": "$ \u2014"}, "Net amount recognized: -- Underfunded status": {"2019": -43.5, "2018": -46.4}, "Net amount recognized: -- Accumulated benefit obligation at end of year": {"2019": 43.5, "2018": 46.4}, "Net amount recognized in consolidated balance sheets consists of: -- Current liability": {"2019": -5.3, "2018": -5.3}, "Net amount recognized in consolidated balance sheets consists of: -- Non-current liability": {"2019": -38.2, "2018": -41.1}, "Net amount recognized in consolidated balance sheets consists of: -- Net amount recognized": {"2019": -43.5, "2018": -46.4}, "Amounts recognized in accumulated other comprehensive loss consist of: -- Net actuarial (gain) loss": {"2019": -0.6, "2018": 0.4}, "Amounts recognized in accumulated other comprehensive loss consist of: -- Prior service credit": {"2019": -2.6, "2018": -3.0}, "Amounts recognized in accumulated other comprehensive loss consist of: -- Total": {"2019": -3.2, "2018": -2.6}}
benefit_obligation_at_beginning_of_period_2018 = df["Change in benefit obligations: -- Benefit obligation at beginning of period"]["2018"]
benefit_obligation_at_beginning_of_period_2019 = df["Change in benefit obligations: -- Benefit obligation at beginning of period"]["2019"]
answer = benefit_obligation_at_beginning_of_period_2018 - benefit_obligation_at_beginning_of_period_2019