What is the increase/ (decrease) in Sales of goods and services for December 31, 2018 to 2019?
{"": {"As of January 1,2018": "NT$(In Thousands)", "As of December 31,2018": "NT$(In Thousands)", "As of December 31,2019": "NT$(In Thousands)"}, "Sales of goods and services": {"As of January 1,2018": 3951414, "As of December 31,2018": 932371, "As of December 31,2019": 1470195}, "Current": {"As of January 1,2018": 3951414, "As of December 31,2018": 932371, "As of December 31,2019": 988115}, "Noncurrent": {"As of January 1,2018": "\u2014", "As of December 31,2018": "\u2014", "As of December 31,2019": 482080}, "Total": {"As of January 1,2018": 3951414, "As of December 31,2018": 932371, "As of December 31,2019": 1470195}}
sales_2019 = df["Sales of goods and services"]["As of December 31,2019"]
sales_2018 = df["Sales of goods and services"]["As of December 31,2018"]
answer = sales_2019 - sales_2018