In Young's double slit experiment the separation d between the slits is 2 mm the wavelength $\lambda$ of the light used is 5896 $\stackrel{o}{A}$ and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is $0.{20}^o$. To increase the fringe angular width to $0.{21}^o$ (with same $\lambda$ and D) the separation between the slits needs to be changed to