Force on Strip in YDSE!

Figure shows two identical narrow slits $S_1$ and $S_2$ .

A very small completely absorbing strip is placed at distance ' $y$ ' from the point $C$ .

' $C$ ' is the point on the screen equidistant from $S_1$ and $S_2$ .

Assume ( $\lambda << d << D$ ) where $\lambda, d$ and $D$ have usual meaning.

When $S_2$ is covered, the force due to light acting on strip is ' $F$ ' and when both slits are opened the force acting on strip is $2F$ . Minimum positive ' $y$ ' ( $<<D$ ) coordinate of the strip in terms of $\lambda, d$ and $D$ is $\frac{\lambda D}{2md}$ .

What is the value of $m$ ?