def fib(n):
    """
    Fibonacci number is a series whose each number is the sum of two 
    previous numbers. Ex: 1,1,2,3,5,8,13,21,34,55...
    """
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fib(n-1) + fib(n-2)