What was the change in onsite supervision costs  from 2018 to 2019 ?
{"Progress shipyard installments": {"2018": 152075, "2019": 197637}, "Onsite supervision costs": {"2018": 5766, "2019": 3879}, "Critical spare parts, equipment and other vessel delivery expenses": {"2018": 1434, "2019": 1807}, "Total": {"2018": 159275, "2019": 203323}}
onsite_supervision_2019 = df["Onsite supervision costs"]["2019"]
onsite_supervision_2018 = df["Onsite supervision costs"]["2018"]
answer = onsite_supervision_2019 - onsite_supervision_2018