What is the increase/ (decrease) in Global Technology Services external revenue from 2017 to 2018
{"($ in millions) -- For the year ended December 31:": {"2018": 2018, "2017": 2017, "Yr.-to-Yr. Percent Change": "Yr.-to-Yr. Percent Change", "Yr.-to-Yr. Percent Change Adjusted for Currency": "Yr.-to-Yr. Percent Change Adjusted for Currency"}, "($ in millions) -- Global Technology Services external revenue": {"2018": 29146.0, "2017": 29213.0, "Yr.-to-Yr. Percent Change": -0.2, "Yr.-to-Yr. Percent Change Adjusted for Currency": -0.8}, "($ in millions) -- Infrastructure & Cloud Services": {"2018": 22185.0, "2017": 22016.0, "Yr.-to-Yr. Percent Change": 0.8, "Yr.-to-Yr. Percent Change Adjusted for Currency": 0.0}, "($ in millions) -- Technology Support Services": {"2018": 6961.0, "2017": 7196.0, "Yr.-to-Yr. Percent Change": -3.3, "Yr.-to-Yr. Percent Change Adjusted for Currency": -3.5}}
global_technology_services_external_revenue_2018 = df["($ in millions) -- Global Technology Services external revenue"]["2018"]
global_technology_services_external_revenue_2017 = df["($ in millions) -- Global Technology Services external revenue"]["2017"]
answer = global_technology_services_external_revenue_2018 - global_technology_services_external_revenue_2017