What is the change in combined pension plan in 2021 from 2020?
{"Estimated future benefit payments: -- 2020": {"Combined Pension Plan": 971, "Post-Retirement Benefit Plans": 242, "Medicare Part D Subsidy Receipts": -6}, "Estimated future benefit payments: -- 2021": {"Combined Pension Plan": 921, "Post-Retirement Benefit Plans": 238, "Medicare Part D Subsidy Receipts": -6}, "Estimated future benefit payments: -- 2022": {"Combined Pension Plan": 893, "Post-Retirement Benefit Plans": 232, "Medicare Part D Subsidy Receipts": -6}, "Estimated future benefit payments: -- 2023": {"Combined Pension Plan": 868, "Post-Retirement Benefit Plans": 226, "Medicare Part D Subsidy Receipts": -5}, "Estimated future benefit payments: -- 2024": {"Combined Pension Plan": 842, "Post-Retirement Benefit Plans": 219, "Medicare Part D Subsidy Receipts": -5}, "Estimated future benefit payments: -- 2025-2029": {"Combined Pension Plan": 3813, "Post-Retirement Benefit Plans": 986, "Medicare Part D Subsidy Receipts": -20}}
combined_pension_plan_2021 = df["Estimated future benefit payments: -- 2021"]["Combined Pension Plan"]
combined_pension_plan_2020 = df["Estimated future benefit payments: -- 2020"]["Combined Pension Plan"]
answer = combined_pension_plan_2021 - combined_pension_plan_2020