Prashant And Sum

Yesterday, $Prashant$ $Singh$ , learned a magically & logically efficient method to find the sum of the integers from $1$ to $N$. He denotes it as $sum(N)$. But today , he defined his own new method: $sum(Z , N)$, which means the operation sum applied $Z$ times: on first $N$ Natural Number, and each subsequent time to the result of the previous operation. For example, if Z = 2 and N = 3, then $s