input: Write a function to find the largest index of an element in an array that is less than or equal to the element immediately preceding it. If there is no such element, return -1. The array will have only unique values.
  assert find_index([1,2,4,3,5])==3
  assert find_index([1,2,4,5])==-1
  assert find_index([1,4,2,5,6,7,8,9,10])==2
  assert find_index([4,8,5,7,3])==4
  assert find_index([])==-1
output: def find_index(arr):
      for i in range(len(arr)-1, 0, -1):
          if arr[i] <= arr[i-1]:
              return i
      return -1