What is the change in PSG Revenue from external customers from year ended December 31, 2018 to 2019?
{"For year ended December 31, 2019:   -- Revenue from external customers": {"PSG": 2788.3, "ASG": 1972.3, "ISG": 757.3, "Total": 5517.9}, "For year ended December 31, 2019:   -- Segment gross profit": {"PSG": 976.0, "ASG": 794.8, "ISG": 275.4, "Total": 2046.2}, "For year ended December 31, 2018:   -- Revenue from external customers": {"PSG": 3038.2, "ASG": 2071.2, "ISG": 768.9, "Total": 5878.3}, "For year ended December 31, 2018:   -- Segment gross profit": {"PSG": 1110.1, "ASG": 878.3, "ISG": 317.1, "Total": 2305.5}, "For year ended December 31, 2017:   -- Revenue from external customers  ": {"PSG": 2819.3, "ASG": 1950.9, "ISG": 772.9, "Total": 5543.1}, "For year ended December 31, 2017:   -- Segment gross profit  ": {"PSG": 959.8, "ASG": 817.8, "ISG": 302.6, "Total": 2080.2}}
revenue_2019 = df["For year ended December 31, 2019:   -- Revenue from external customers"]["PSG"]
revenue_2018 = df["For year ended December 31, 2018:   -- Revenue from external customers"]["PSG"]
answer = revenue_2019 - revenue_2018