def _c(self, s1, s2):
        """Normalization of the LDF

        As given in Montanus, paper to follow.

        :param s1: shower shape parameter.
        :param s2: shower shape parameter.
        :return: c(s1,s2)

        """
        r0 = self._r0
        return (gamma(-s2) /
                (2 * pi * r0 ** 2 * gamma(s1 + 2) * gamma(-s1 - s2 - 2)))