from math import sqrt

fi = (1 + sqrt(5))/2
psi = (1 - sqrt(5))/2

def F(n):
    return 7 * (fi**n)

# print(fi**20 - (fi**19 + fi**18))
# print(psi**20 - (psi**19 + psi**18))
# print(F(20) - (F(19) + F(18)))

def ex_2(n):
    a = (7 + 4j)/2j
    b = (-7 + 4j)/2j
    return a * ((0+1j)**n) + b * ((0-1j)**n)