input: Write a python function to find the missing number in a list containing a range of consecutive numbers.
  assert find_missing_number([3, 4, 5, 7, 8]) == 6
  assert find_missing_number([10, 11, 13, 14]) == 12
  assert find_missing_number([1, 2, 3, 5]) == 4
output: def find_missing_number(numbers):
      n = len(numbers) + 1
      expected_sum = (n * (numbers[0] + numbers[-1])) // 2
      actual_sum = sum(numbers)
      return expected_sum - actual_sum