What is the nominal difference between the sales in F19 and F18?
{"Sales": {"53 WEEKS": 3797.0, "52 WEEKS": 3566.0, "CHANGE": 6.5, "NORMALISED": 4.2}, "LBITDA before -- significant items": {"53 WEEKS": -5.0, "52 WEEKS": -30.0, "CHANGE": -82.2, "NORMALISED": -88.7}, "LBITDA before -- Depreciation and amortisation": {"53 WEEKS": -80.0, "52 WEEKS": -80.0, "CHANGE": 0.7, "NORMALISED": 0.7}, "LBITDA before -- LBIT before significant items": {"53 WEEKS": -85.0, "52 WEEKS": -110.0, "CHANGE": -22.2, "NORMALISED": -24.0}, "LBITDA before -- Significant items": {"53 WEEKS": -371.0, "52 WEEKS": "\u2013", "CHANGE": "n.m.", "NORMALISED": "n.m."}, "LBITDA before -- LBIT after significant items": {"53 WEEKS": -456.0, "52 WEEKS": -110.0, "CHANGE": 315.5, "NORMALISED": 313.7}, "LBITDA before -- Gross margin (%)": {"53 WEEKS": 31.1, "52 WEEKS": 31.7, "CHANGE": -59.0, "NORMALISED": -49.0}, "LBITDA before -- Cost of doing business (%)": {"53 WEEKS": 33.4, "52 WEEKS": 34.8, "CHANGE": -142.0, "NORMALISED": -132.0}, "LBITDA before -- LBIT 2 to sales (%)": {"53 WEEKS": -2.3, "52 WEEKS": -3.1, "CHANGE": 83.0, "NORMALISED": 84.0}, "LBITDA before -- Sales per square metre ($)$)": {"53 WEEKS": 3629.0, "52 WEEKS": 3369.0, "CHANGE": 7.7, "NORMALISED": 5.4}, "LBITDA before -- Funds employed": {"53 WEEKS": 204.0, "52 WEEKS": 502, "CHANGE": -59.4, "NORMALISED": ""}, "LBITDA before -- ROFE (%)": {"53 WEEKS": -23.0, "52 WEEKS": -23.3, "CHANGE": 24.0, "NORMALISED": 77.0}}
sales_2019 = df["Sales"]["53 WEEKS"]
sales_2018 = df["Sales"]["52 WEEKS"]
answer = sales_2019 - sales_2018