What is the difference in the net cash provided by investing activities between 2018 and 2019?
{"": {"2019": "", "2018": "", "2017": "(in thousands)", "2016": "", "2015": ""}, "Net cash provided by (used in) operating activities (1)": {"2019": -156832, "2018": 113207, "2017": 64241, "2016": 138720, "2015": 141257}, "Purchase of property and equipment": {"2019": -36531, "2018": -52880, "2017": -89160, "2016": -78640, "2015": -30566}, "Free cash flow": {"2019": -193363, "2018": 60327, "2017": -24919, "2016": 60080, "2015": 110691}, "Net cash provided by (used in) investing activities": {"2019": 25761, "2018": 17496, "2017": -28718, "2016": -392666, "2015": -170027}, "Net cash provided by (used in) financing activities (1)": {"2019": -8406, "2018": 1287, "2017": 4635, "2016": 19794, "2015": 368953}}
net_cash_investing_2019 = df["Net cash provided by (used in) investing activities"]["2019"]
net_cash_investing_2018 = df["Net cash provided by (used in) investing activities"]["2018"]
answer = net_cash_investing_2019 - net_cash_investing_2018