def fib_num(A):
    for i in range(2, A+1):
        x=number(n)
        if x==A:
            return n
            break
        if x>A:
            return '-1'
            break

def number(n):
    if n <= 2:
        return 1
    else:
        return fib(n-1) + fib(n-2)