What was the change in Accumulated benefit obligation between 2018 and 2019?
{"Projected benefit obligation": {"2019": 174690, "2018": 161104}, "Accumulated benefit obligation": {"2019": 161729, "2018": 152380}, "Fair value of plan assets": {"2019": 158101, "2018": 151715}}
accumulated_benefit_obligation_2019 = df["Accumulated benefit obligation"]["2019"]
accumulated_benefit_obligation_2018 = df["Accumulated benefit obligation"]["2018"]
answer = accumulated_benefit_obligation_2019 - accumulated_benefit_obligation_2018