input: 
  from typing import List
  
  def avg_relative_diff(numbers: List[float], k: int) -> float:
      """ Given a list of numbers and an integer k, calculate the average relative difference between each element and the element k positions ahead of it. 
      If the list length is less than k + 1, return 0.0.
      >>> avg_relative_diff([1.0, 2.0, 3.0, 4.0], 1)
      0.3333333333333333
      >>> avg_relative_diff([10.0, 12.0, 5.0, 8.0, 6.0, 9.0], 2)
      0.0
      """
  
output:     if len(numbers) < k + 1:
          return 0.0
      
      total_diff = 0.0
      count = 0
      
      for i in range(len(numbers) - k):
          diff = abs(numbers[i] - numbers[i + k]) / numbers[i]
          total_diff += diff
          count += 1
      
      return total_diff / count