Android : how can i get exact opengl cordinates from touch in android opengl

Hey i'm new to OpenGL i'm using the opengl ES 2.0 Example in the android developers site.

I'm getting the screen cordinates of using the touch events in android.

public boolean onTouchEvent(MotionEvent e) {

    float x = e.getX();
    float y = e.getY();

}

can anyone please help me in converting them into opneGL world coordinates.

thanks in Advance