input: 
  def find_index(arr: List[int], x: int) -> int:
      """Find the index of a given value in the array.
      If the value is not present, return -1.
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([2, 5, 9, 15], 20)
      -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