A Young’s double slit interference arrangement with slits  $S_1$ in air and  $S_2$ is immersed in water (refractive index $= 3/2$) 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