input: 
  def find_index(arr: list[int], x: int) -> int:
      """
      Find the index of a given value in an array. If the value is not found, return -1.
      
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 10)
      -1
      """
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
  
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1