What was the change in the investments into Novasentis between 2018 and 2019?
{"Nippon Yttrium Co., Ltd (\"NYC\")": {"2019": 8215, "2018": 8148}, "NT Sales Co., Ltd (\"NTS\")": {"2019": 1218, "2018": 998}, "Novasentis": {"2019": 977, "2018": 2870}, "KEMET Jianghai Electronics Components Co., Ltd (KEMET Jianghai)": {"2019": 2515, "2018": "\u2014"}, "": {"2019": 12925, "2018": 12016}}
novasentis_2019 = df["Novasentis"]["2019"]
novasentis_2018 = df["Novasentis"]["2018"]
answer = novasentis_2019 - novasentis_2018