<s>[INST] def count_letter_combinations(letters, target_sum):
    """
    This function counts the number of combinations of letters that add up to the target sum.
    
    Parameters:
    letters (str): A string of letters
    target_sum (int): The target sum that the combinations should add up to
    
    Returns:
    int: The number of combinations of letters that add up to the target sum
    """[/INST]