Previous Fibonacci

2022-06-20.py

question = """

Given a Fibonacci number, give the previous Fibonacci number. 
If the number given is not a Fibonacci number, return -1.

"""

def prevFib(number):
    fibs = [ 0, 1 ]
    answer = None
    while True:
        next_fib = fibs[-1] + fibs[-2]
        if next_fib == number:
            answer = fibs[-1]
            break
        elif next_fib > number:
            answer = -1
            break
        fibs.append(next_fib)
    print(answer)

prevFib(5)
prevFib(6)