code: def calc_check_digit(digits):
          
          ints = [int(d) for d in digits]
          l = len(ints)
          odds = slice((l - 1) % 2, l, 2)
          even = slice(l % 2, l, 2)
          checksum = 3 * sum(ints[odds]) + sum(ints[even])
          return str(-checksum % 10)
docs: Calculate and return the GS1 check digit.