Returns the checksum of CPF digits.
    References to the algorithm:
    https://pt.wikipedia.org/wiki/Cadastro_de_pessoas_f%C3%ADsicas#Algoritmo
    https://metacpan.org/source/MAMAWE/Algorithm-CheckDigits-v1.3.0/lib/Algorithm/CheckDigits/M11_004.pm

def checksum(digits):
    """
    Returns the checksum of CPF digits.
    References to the algorithm:
    https://pt.wikipedia.org/wiki/Cadastro_de_pessoas_f%C3%ADsicas#Algoritmo
    https://metacpan.org/source/MAMAWE/Algorithm-CheckDigits-v1.3.0/lib/Algorithm/CheckDigits/M11_004.pm
    """
    s = 0
    p = len(digits) + 1
    for i in range(0, len(digits)):
        s += digits[i] * p
        p -= 1

    reminder = s % 11
    if reminder == 0 or reminder == 1:
        return 0
    else:
        return 11 - reminder