A young's double slit interference arrangement with slits $S_1$ and $S_2$ is immersed in water (refractive index $=4 / 3)$ as shown in the figure. The positions of maxima on the surface of water are given by $x^2=p^2 m^2 \lambda^2-d^2$, where $\lambda$ is the wavelength of light in air (refractive index $=1$ ), $2 d$ is the separation between the slits and $m$ is an integer. The value of $p$ is