What is the 2019 average sales of goods and services to associates ?
{"": {"2019": "\u20acm", "2018": "\u20acm", "2017": "\u20acm"}, "Sales of goods and services to associates": {"2019": 27, "2018": 19, "2017": 37}, "Purchase of goods and services from associates": {"2019": 3, "2018": 1, "2017": 90}, "Sales of goods and services to joint arrangements": {"2019": 242, "2018": 194, "2017": 19}, "Purchase of goods and services from joint arrangements": {"2019": 192, "2018": 199, "2017": 183}, "Net interest income receivable from joint arrangements1": {"2019": 96, "2018": 120, "2017": 87}, "Trade balances owed: -- by associates": {"2019": 1, "2018": 4, "2017": "\u2013"}, "Trade balances owed: -- to associates": {"2019": 3, "2018": 2, "2017": 1}, "Trade balances owed: -- by joint arrangements": {"2019": 193, "2018": 107, "2017": 158}, "Trade balances owed: -- to joint arrangements": {"2019": 25, "2018": 28, "2017": 15}, "Trade balances owed: -- Other balances owed by joint arrangements1": {"2019": 997, "2018": 1328, "2017": 1209}, "Trade balances owed: -- Other balances owed to joint arrangements1": {"2019": 169, "2018": 150, "2017": 127}}
sales_2019 = df["Sales of goods and services to associates"]["2019"]
sales_2018 = df["Sales of goods and services to associates"]["2018"]
trade_balances_2018 = df["Trade balances owed: -- to associates"]["2018"]
answer = (sales_2019 + sales_2018) / trade_balances_2018