code: def checksum(digits):
      
      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
docs: 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