In Young's double slit experiment, two slits $S_{1}$ and $S_{2}$ are '$d$' distance apart and the separation from slits to screen is $\mathrm{D}$ (as shown in figure). Now if two transparent slabs of equal thickness $0.1 \mathrm{~mm}$ but refractive index $1.51$ and $1.55$ are introduced in the path of beam $(\lambda=4000$ $\mathop A\limits^o $) from $\mathrm{S}_{1}$ and $\mathrm{S}_{2}$ respectively. The central bright fringe spot will shift by ___________ number of fringes.