input: Write a python function that finds the largest index of an element in the provided array which is not greater than the element that comes just before it. If no such element is found, the function should return -1. The array will have unique values.
  assert can_reorder([1, 2, 4, 3, 5]) == 3
  assert can_reorder([1, 2, 4, 5]) == -1
  assert can_reorder([1, 4, 2, 5, 6, 7, 8, 9, 10]) == 2
  assert can_reorder([4, 8, 5, 7, 3]) == 4
  assert can_reorder([]) == -1
output: def can_reorder(arr):
      for i in range(len(arr) - 1, 0, -1):
          if arr[i] <= arr[i - 1]:
              return i
      return -1