def swap_score(xs, ys):
    """Return the number of characters that needs to be substituted
    to change the characters in the first string into the second string.

    If the strings not of equal length, we will disregard all
    "extra" characters in the larger string.

    This function only considers the leftmost character of each string to
    correspond to each other. For example, if the two inputs are
    "word" and "weird", we'll only consider potential substitutions
    for "word" and "weir".
    """
    return sum(x != y for x, y in zip(xs, ys))