In a Young's double slit experiment, a combination of two glass wedges $$A$$ and $$B$$, having refractive indices 1.7 and 1.5, respectively, are placed in front of the slits, as shown in the figure. The separation between the slits is $$d = 2$$ mm and the shortest distance between the slits and the screen is $$D = 2$$ m. Thickness of the combination of the wedges is $$t = 12 \, \mu$$m. The value of $$l$$ as shown in the figure is 1 mm. Neglect any refraction effect at the slanted interface of the wedges. Due to the combination of the wedges, the central maximum shifts (in mm) with respect to O by ______.