def string_index(neg_idx, string):
    """Solution to exercise R-1.8.

    Python allows negative integers to be used as indices into a sequence,
    such as a string. If string s has length n, and expression s[k] is used for
    index −n ≤ k < 0, what is the equivalent index j ≥ 0 such that s[j]
    references the same element?
    """
    n = len(string)
    return n + neg_idx