Custom Field: Number

I want to add a custom field that takes the input just number.
Actually this input is to be used as an input for roll_number of student.

And also how to make it a “required” field?

Currently, what I have so far:
Roll No.<input type=”number” name=”roll_no” required min=”0″>

But that’s not working as it should. If I press submit button without filling the fields, all fields are highlighted in red except this custom one. How this must be done?

http://wordpress.org/plugins/contact-forms/