Figure shows a long thread along the axis of a long hollow cylinder. The uniform charge per unit length of thread is  $\lambda$  while uniform surface charge density of the cylinder is $-\sigma$. The radius of the cylinder is R. Total flux associated with a coaxial cylinder of finite length and radius twice that of the charged cylinder is zero, if the magnitude of  $\sigma =\frac{\lambda }{n\pi R}$  then n is