What was the change in money market funds between 2018 and 2019?
{"Cash": {"2019": 437, "2018": 268}, "Foreign government treasury bills": {"2019": 37, "2018": 32}, "Money market funds": {"2019": 5320, "2018": 3925}, "Cash and cash equivalents": {"2019": 5794, "2018": 4225}}
money_market_funds_2019 = df["Money market funds"]["2019"]
money_market_funds_2018 = df["Money market funds"]["2018"]
answer = money_market_funds_2019 - money_market_funds_2018