input: Write a Python function that finds and returns the last index of an element in the input list, which is less than or equal to the element directly before it. If no such element can be found, the function should return -1. The input list will have unique values.
  assert can_rearrange([1,2,4,3,5])==3
  assert can_rearrange([1,2,4,5])==-1
  assert can_rearrange([1,4,2,5,6,7,8,9,10])==2
  assert can_rearrange([4,8,5,7,3])==4
  assert can_rearrange([])==-1
output: def can_rearrange(lst):
      for i in range(len(lst)-1, 0, -1):
          if lst[i] <= lst[i-1]:
              return i
      return -1