Xpath input field containing text inserted

I have the following problem:

I have to check via Selenium if an input field has been filled with a expected manually inserted text.

Here's the input field with the value of '50':

<input type="text" class="value_per form-control input-sm valid" name="per_slider1" idslider="1" value="50">

I won't look for the @value, but instead, the value that I can edit in the form. For clarification, the input's @value will be always '50' even if I edit the input text to '100' and I don't know how to get the '100' via Xpath.

I already tried with:

//input[@name='per_slider']/text()

or

//input[@name='per_slider']/@value

but they don't work.

If you have a fast solution for this, all comments are appreciated.

I'm using PHPunit to test a website via Selenium(This info should be useless).