What was the difference in the minimum revenue from dry bulk vessels between 2020 and 2021?
{"2020": {"Dry bulk vessels": 22266, "Logistics business": 129437}, "2021": {"Dry bulk vessels": 4607, "Logistics business": 97544}, "2022": {"Dry bulk vessels": 3445, "Logistics business": 75425}, "2023": {"Dry bulk vessels": "\u2014", "Logistics business": 69250}, "2024": {"Dry bulk vessels": "\u2014", "Logistics business": 60200}, "2025 and thereafter": {"Dry bulk vessels": "\u2014", "Logistics business": 642479}, "Total minimum revenue, net of commissions": {"Dry bulk vessels": 30318, "Logistics business": 1074335}}
dry_bulk_vessels_2020 = df["2020"]["Dry bulk vessels"]
dry_bulk_vessels_2021 = df["2021"]["Dry bulk vessels"]
answer = dry_bulk_vessels_2020 - dry_bulk_vessels_2021