What is the increase / (decrease) in the receivables from 31 December 2018 to 31 December 2019?
{"Receivables (1)": {"At December 31, 2019": 5752, "At December 31, 2018": 5448, "At January 1, 2018": 5555}, "Device payment plan agreement receivables (2)": {"At December 31, 2019": 15313, "At December 31, 2018": 12272, "At January 1, 2018": 2073}, "Contract assets": {"At December 31, 2019": 761, "At December 31, 2018": 772, "At January 1, 2018": 858}, "Contract liabilities": {"At December 31, 2019": 4721, "At December 31, 2018": 4521, "At January 1, 2018": 3445}}
receivables_1_2019 = df["Receivables (1)"]["At December 31, 2019"]
receivables_1_2018 = df["Receivables (1)"]["At December 31, 2018"]
answer = receivables_1_2019 - receivables_1_2018