What was the change in the Net investment hedging instruments between 2018 and 2019?
{"Derivatives designated as hedging instruments: -- Foreign currency derivativescash flow hedges": {"July 27, 2019": 663, "July 28, 2018": 147}, "Derivatives designated as hedging instruments: -- Interest rate derivatives": {"July 27, 2019": 4500, "July 28, 2018": 6750}, "Derivatives designated as hedging instruments: -- Net investment hedging instruments": {"July 27, 2019": 309, "July 28, 2018": 250}, "Derivatives not designated as hedging instruments: -- Foreign currency derivatives": {"July 27, 2019": 2708, "July 28, 2018": 2298}, "Derivatives not designated as hedging instruments: -- Total return swapsdeferred compensation": {"July 27, 2019": 574, "July 28, 2018": 566}, "Derivatives not designated as hedging instruments: -- Total": {"July 27, 2019": 8754, "July 28, 2018": 10011}}
net_investment_2019 = df["Derivatives designated as hedging instruments: -- Net investment hedging instruments"]["July 27, 2019"]
net_investment_2018 = df["Derivatives designated as hedging instruments: -- Net investment hedging instruments"]["July 28, 2018"]
answer = net_investment_2019 - net_investment_2018