What was the change in the fair value of plan assets between 2018 and 2019?
{"Fair value of plan assets": {"April 26, 2019": 31, "April 27, 2018": 25}, "Benefit obligations": {"April 26, 2019": -61, "April 27, 2018": -53}, "Unfunded obligations": {"April 26, 2019": -30, "April 27, 2018": -28}}
fair_value_2019 = df["Fair value of plan assets"]["April 26, 2019"]
fair_value_2018 = df["Fair value of plan assets"]["April 27, 2018"]
answer = fair_value_2019 - fair_value_2018