def _len_lcs(x, y):
    """
    Returns the length of the Longest Common Subsequence between sequences x
    and y.
    Source: http://www.algorithmist.com/index.php/Longest_Common_Subsequence

    :param x: sequence of words
    :param y: sequence of words
    :returns integer: Length of LCS between x and y
    """
    table = _lcs(x, y)
    n, m = _get_index_of_lcs(x, y)
    return table[n, m]