What was the change in the fair value of plans' assets?
{"": {"2019": "$ million", "2018": "$ million"}, "At 1 January": {"2019": 254.2, "2018": 282.6}, "Interest income on plans assets": {"2019": 7.0, "2018": 7.0}, "Employer contributions": {"2019": 6.6, "2018": 6.8}, "Benefit payments": {"2019": -11.4, "2018": -12.1}, "Plan administration expenses": {"2019": -0.6, "2018": -0.5}, "Re-measurement gain/(loss) on plans assets": {"2019": 25.8, "2018": -14.2}, "Exchange adjustment": {"2019": 9.5, "2018": -15.4}, "Fair value of plans assets": {"2019": 291.1, "2018": 254.2}}
fair_value_2019 = df["Fair value of plans assets"]["2019"]
at_1_january_2019 = df["At 1 January"]["2019"]
answer = fair_value_2019 - at_1_january_2019