Let $X_{1}, \ldots, X_{m}$ be independent nonnegative integer-valued random variables all having the same distribution $\left\{p_{n}, n \geq 0\right\}$; and $r_{n}=$ $\sum_{k=n}^{\infty} p_{k} .$ Show that
$$
E\left\{\min \left(X_{1}, \ldots, X_{m}\right)\right\}=\sum_{n=1}^{\infty} r_{n}^{m}
$$