How to show that (X,d) is complete iff every sequence ($x_n$) in X with $\sum_{n=1}^\infty$d($x_{n+1},x_{n}$) < $\infty$,is convergent?

Question

Show that (X,d) is complete iff every sequence ($x_n$) in X with $\sum_{n=1}^\infty$d($x_{n+1},x_{n}$) < $\infty$,is convergent.

I know that I have to provide what i have tried. But actually i dont find any way to start. Since d($x_{n+1},x_{n}$) converges to 0 as n$\rightarrow$$\infty$ does not imply ($x_n$) is Cauchy. That's where i stuck. If (X,d) is complete so each Cauchy sequence converges to some point in x. But i do not know how to use this facts to show the rest.

Show that a metric space $(X,d)$ is complete iff every sequence $(x_n)$ in $X$ with $\sum_{n=1}^\infty d(x_{n+1},x_n) < \infty$, is convergent.

The link provides a solution of this question. I saw that i dont understand what i have to do.