How to compute the limit: $lim _{ nto infty } sum _{ r=1 }^{ n }{ frac { [2rx] }{ { n }^{ 2 } } } $

How to compute the limit:
$\lim _{ n\to \infty } \sum _{ r=1 }^{ n }{ \frac { [2rx] }{ { n }^{ 2 } } } $

I'm trouble in handling the [.] greatest integer function.Help please!