input: Write a function to find the missing number in a list of consecutive numbers.
  assert missing_number([4, 5, 7, 9, 10]) == 6
  assert missing_number([1, 2, 4, 5]) == 3
  assert missing_number([10, 11, 12, 14]) == 13
output: def missing_number(numbers):
      n = len(numbers)
      total_sum = (n + 1) * (numbers[0] + numbers[-1]) // 2
      actual_sum = sum(numbers)
      return total_sum - actual_sum