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 $D$ (as shown in figure). Now if two transparent slabs of equal thickness 0.1 mm but refractive index 1.51 and 1.55 are introduced in the path of beam $\lambda = 4000~\AA$ from $S_1$ and $S_2$ respectively. The central bright fringe spot will shift by $\_\_\_\_$ number of fringes.