Two light waves of wavelengths 800 and $600 \mathrm{~nm}$ are used in Young's double slit experiment to obtain interference fringes on a screen placed $7 \mathrm{~m}$ away from plane of slits. If the two slits are separated by $0.35 \mathrm{~mm}$, then shortest distance from the central bright maximum to the point where the bright fringes of the two wavelength coincide will be ______ $\mathrm{mm}$.