input: Develop a Python function that identifies the missing element in a sequential array.
  assert find_missing([1, 2, 3, 5], 4) == 4
  assert find_missing([1, 3, 4, 5], 4) == 2
  assert find_missing([1, 2, 3, 5, 6, 7], 5) == 4
output:     expected_sum = sum(range(1, n+1))
      actual_sum = sum(arr)
      return expected_sum - actual_sum