error: attribute layout_heightPercent not found

 1 <?xml version="1.0" encoding="utf-8"?>
 2 <android.support.percent.PercentFrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
 3     xmlns:app="http://schemas.android.com/apk/res-auto"
 4     android:layout_width="match_parent"
 5     android:layout_height="match_parent">
 6     <Button
 7         android:id="@+id/button1"
 8         android:layout_gravity="left|top"
 9         android:text="Button 1"
10         app:layout_heightPercent="30%"
11         app:layout_widthPercent="20%"
12         />
13 
14     <Button
15         android:id="@+id/button2"
16         android:layout_gravity="right|top"
17         android:text="Button 2"
18         app:layout_heightPercent="30%"
19         app:layout_widthPercent="80%" />
20 
21     <Button
22         android:id="@+id/button3"
23         android:layout_gravity="left|bottom"
24         android:text="Button 3"
25         app:layout_heightPercent="70%"
26         app:layout_widthPercent="50%" />
27 
28     <Button
29         android:id="@+id/button4"
30         android:layout_gravity="right|bottom"
31         android:text="Button 4"
32         app:layout_heightPercent="70%"
33         app:layout_widthPercent="50%" />
34 
35 
36 </android.support.percent.PercentFrameLayout>