def difference(array, k=1):
    """Calculate l[n] - l[n-k]
    """
    if (len(array) - k) < 1:
        raise ValueError()
    if k < 0:
        raise ValueError("k has to be greater or equal than zero!")
    elif k == 0:
        return [i - i for i in array]
    else:
        return [j - i for i, j in zip(array[:-k], array[k:])]