Error in an XML file: aborting build for my new app!

Hello guys,
it may sound an easy question but I have trying to fix it for a couple of days and no result yet.
This is my very first app and I get this error no matter what I do.
here is the source code for my app.
In “activity_main.xml” file I have :


<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="16dp"
    android:paddingLeft="64dp"
    android:paddingRight="64dp"
    android:paddingTop="16dp"
    tools:context="com.davidgassner.myfirstapplication.MainActivity" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/F_app" />

    <Button
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/F_B" />

</RelativeLayout>

and here is the source code in strings.xml


<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:android="http://schemas.android.com/apk/res/android">

    <string name="app_name">My First Application</string>
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
    <string name="F_app">This is my first android application!</string>
    <string name="F_B">And this is a clickable button!</string>

</resources>

Notes:

  1. I do not have any .out.xml file in res/layout.
  2. I have added the
<?xml version="1.0" encoding="utf-8"?> 

to the beginning of the activity_main.xml file but the error is still there.

I really need your help to go forward with programming.
Many thanks

Try cleaning your project