<Grid
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
              <Canvas>
                <Path Fill="LightBlue" Stroke="Black" StrokeThickness="2">
                    <Path.Data>
                        <CombinedGeometry GeometryCombineMode="Exclude">
                            <CombinedGeometry.Geometry1>
                                <RectangleGeometry Rect="50,50,700,550"/>
                            </CombinedGeometry.Geometry1>
                            <CombinedGeometry.Geometry2>
                                <EllipseGeometry Center="150,125" RadiusX="150" RadiusY="150"/>
                            </CombinedGeometry.Geometry2>
                        </CombinedGeometry>
                    </Path.Data>
                </Path>
            </Canvas>
</Grid>