What is the increase/ (decrease) in OEM from the period December 31, 2018 to 2019?
{"": {"December 31, 2019": "", "September 29, 2019": "(Unaudited, in %)", "December 31, 2018": ""}, "OEM": {"December 31, 2019": 72, "September 29, 2019": 72, "December 31, 2018": 69}, "Distribution": {"December 31, 2019": 28, "September 29, 2019": 28, "December 31, 2018": 31}, "Total": {"December 31, 2019": 100, "September 29, 2019": 100, "December 31, 2018": 100}}
oem_december_31_2019 = df["OEM"]["December 31, 2019"]
oem_december_31_2018 = df["OEM"]["December 31, 2018"]
answer = oem_december_31_2019 - oem_december_31_2018