input: Write a python function to find the missing number in an array of consecutive integers.
  assert missing_number([1, 2, 4, 5]) == 3
  assert missing_number([1, 3, 4, 5]) == 2
  assert missing_number([2, 3, 4, 5, 6, 8, 9]) == 7
output: def missing_number(arr):
      n = len(arr) + 1
      total_sum = n * (n + 1) // 2
      actual_sum = sum(arr)
      return total_sum - actual_sum