In fiscal year 2018, what is the percentage constitution of revenues from FEI-Zyfer among the total revenue?
{"FEI-NY": {"2019": 38096, "2018": 26936, "$": 11160, "%": 41}, "FEI-Zyfer": {"2019": 12235, "2018": 15272, "$": -3037, "%": -20}, "Intersegment sales": {"2019": -822, "2018": -2801, "$": 1979, "%": -71}, "": {"2019": 49509, "2018": 39407, "$": 10102, "%": 26}}
fei_zyfer_2018 = df["FEI-Zyfer"]["2018"]
total_revenue_2018 = df[""]["2018"]
answer = fei_zyfer_2018 / total_revenue_2018 * 100.0