input: Write a function to find the missing number in a given list of consecutive numbers.
  assert find_missing_number([1, 2, 3, 4, 6, 7, 8, 9]) == 5
  assert find_missing_number([10, 11, 12, 14, 15, 16]) == 13
  assert find_missing_number([100, 101, 102, 104, 105]) == 103
output: def find_missing_number(numbers):
      n = len(numbers) + 1
      total_sum = (n * (numbers[0] + numbers[-1])) // 2
      actual_sum = sum(numbers)
      return total_sum - actual_sum