input: Write a Python function to locate the absent element in a sorted list.
  assert find_missing_item([1,2,3,5],4) == 4
  assert find_missing_item([1,3,4,5],4) == 2
  assert find_missing_item([1,2,3,5,6,7],5) == 4
output: def find_missing_item(arr, n):
      expected_sum = n * (n + 1) // 2
      actual_sum = sum(arr)
      return expected_sum - actual_sum