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), 2d is the separation between the slits and m is an integer. The value of p is