What is the percentage increase in marketable securities from 2018 to 2019? 
{"": {"2019": "", "2018": "(in thousands)", "2017": ""}, "Cash and cash equivalents": {"2019": 36617, "2018": 60444, "2017": 67495}, "Marketable securities": {"2019": 434761, "2018": 339424, "2017": 297115}, "Working capital": {"2019": 368912, "2018": 370445, "2017": 361621}, "Net cash provided by operating activities": {"2019": 72819, "2018": 83085, "2017": 61893}, "Net cash used in investing activities": {"2019": -103579, "2018": -56237, "2017": -207907}, "Net cash provided by (used in) financing activities": {"2019": 6933, "2018": -33899, "2017": 5477}}
marketable_securities_2019 = df["Marketable securities"]["2019"]
marketable_securities_2018 = df["Marketable securities"]["2018"]
answer = ( marketable_securities_2019 - marketable_securities_2018 ) / marketable_securities_2018 * 100.0