What was the difference in the net book value between trade name and port terminal operating rights?
{"Trade name": {"Acquisition Cost": 100420, "Accumulated Amortization": -47966, "Transfer/ Write off": "$\u2014", "Net Book Value December 31, 2018": 52454}, "Port terminal operating rights": {"Acquisition Cost": 53152, "Accumulated Amortization": -11838, "Transfer/ Write off": "\u2014", "Net Book Value December 31, 2018": 41314}, "Customer relationships": {"Acquisition Cost": 35490, "Accumulated Amortization": -19520, "Transfer/ Write off": "\u2014", "Net Book Value December 31, 2018": 15970}, "Favorable lease terms(**)": {"Acquisition Cost": 32492, "Accumulated Amortization": -2143, "Transfer/ Write off": -1150, "Net Book Value December 31, 2018": 29199}, "Total Intangible assets": {"Acquisition Cost": 221554, "Accumulated Amortization": -81467, "Transfer/ Write off": -1150, "Net Book Value December 31, 2018": 138937}}
trade_name_net_book_value = df["Trade name"]["Net Book Value December 31, 2018"]
port_terminal_net_book_value = df["Port terminal operating rights"]["Net Book Value December 31, 2018"]
answer = trade_name_net_book_value - port_terminal_net_book_value