Planck constant is very familiar in quantum physics. Planck pointed out that if one takes $h$ and Newton's gravitational constant $G$ and the speed of light $c$ as fundamental quantities, it is possible to combine them to form three new independent quantities to replace the customary units of mass, length, and time. If we want to express the Planck time $t_p$ by using these constants what will be the power of $c$ (the speed of light)?