import math


def _get_Rost_ID_threshold(L, n=5):
    """
    This function returns the Rost sequence identity threshold for a given alignment of
    length "L".
    """    
    return n + (480 * pow( L, float('-0.32') *\
        (1 + pow(float(repr(math.e)), float(repr(float(-L) / 1000))))))