Show that sample mean diverges almost surely if the samples are i.i.d. sequences and one of the expectation of samples is infinite.

Question

Let $X_1, X_2, \cdots, X_n$ be an i.i.d. sequence with $E(X_1)=\infty$. I have to show that $\lim_{n \to \infty}\frac{1}{n}\sum_{k=1}^nX_k=\infty$, but I don't know how to solve it... Please help me..