A light beam of frequency $1.2 \times 10^{15} \mathrm{~Hz}$ is incident on a metal in a photoelectric effect experiment. The corresponding maximum kinetic energy of the ejeted photoelectrons from the metal is $6.63 \times 10^{-19} \mathrm{~J}$. The characteristic cut-off frequency of the metal is _______ $\text{Hz}$.