In the arrangement shown in the Fig., light of wavelength $6000\stackrel{\circ }{A}$ is incident on slits $S_1 \text{and} S_2$. Slits $S_3 \text{and} S_4$ have been opened such that $S_3$ is the position of first maximum above the central maximum and $S_4$ is the closest position where intensity is same as that of the light used, below the central maximum. The point $O$ is equidistant from $S_1 \text{and} S_2$ and $O^{\text{'}}$ is equidistant from $S_3 \text{and} S_4$. The intensity light is $I_{0.}$

Find: the intensity of the brightest fringe.