A non-conducting right circular hollow cone of base radius R has a uniform surface charge density $\sigma$ . A part ABP is cut and removed from the cone. The potential due to the remaining portion of the cone at the apex point ‘P’ is  $\frac{x}{y}\frac{\sigma R}{{\in }_0}$, where x, y are positive integers and co-primes. Find the value of  $y-x$ ?