input: Write a function to calculate the difference between the lowest and highest values in an array of integers. If the array is empty, return -1.
  
  def diffHighestLowest(arr):
      if not arr:
          return -1
      lowest = min(arr)
      highest = max(arr)
      return highest - lowest
  
  assert diffHighestLowest([12, 24, 3, 8, -1]) == 25
  assert diffHighestLowest([5, 5, 5, 5]) == 0
  assert diffHighestLowest([-3, -10, -1, -5]) == 9
  assert diffHighestLowest([]) == -1
  assert diffHighestLowest([10]) == 0
output: def diffHighestLowest(arr):
      if not arr:
          return -1
      lowest = min(arr)
      highest = max(arr)
      return highest - lowest