I have a problem and I can not figure it out,

Let c be a fixed constant, $\displaystyle c>0$. Then $\displaystyle \mathbb{E}(|X|)<\infty$ if and only if $\displaystyle \sum_{n=1}^\infty\mathbb{P}(|X|\ge cn)<\infty$. In particular, if the last series converges for one value of c, it converges for all values of c.

I appreciate any help.