def zscore(xi: float, mean: float, std: float) -> float:
    """
    Return the z-score for a single value.
    
    Args:
        xi (float): the single value
        mean (float): mean value from the sequence
        std (float): standart deviation from the sequence
    
    Returns:
        float: the z-score for a single value
    """
    if std != 0:
        return (xi - mean)/std
    else:
        return xi - mean