Let $S$ be a finite set with $m$ elements in $\mathbb{P}^n$. Associate to $S$ the subscheme $Y$ such that the ideal sheaf $\mathcal{I}_Y$ of $Y$ is the ideal sheaf of $S$ to the power $t$, where $t\geq 1$. Let $d\geq 1$. How to see that $$ h^0(\mathbb{P}^n,\mathcal{O}_Y(d) ) = m\left( \begin{array}{c} t+n-1\\ n \end{array} \right). $$
Thanks!!