Android layout error: no resource identifier found for attribute 'lineWidth' in package

I already import viewpagerindicator, but when I make the project, the complier show the error. The error message is : error: No resource identifier found for attribute 'lineWidth' in package 'com.example.sample' My use IDE is Intellij. How can I fix this problem? Thanks.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
    xmlns:app="http://ift.tt/GEGVYd"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.viewpagerindicator.LinePageIndicator
        android:id="@+id/indicator"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_marginBottom="3dp"
        app:lineWidth="25dp" />

</RelativeLayout>