wpf中把按钮变成圆角

<Button x:Name="btn" Content="改变" Width="100" Height="50" Margin="67,197,350,73">
<Button.Template >
<ControlTemplate TargetType="{x:Type Button}" >
<Border BorderBrush="{TemplateBinding Control.BorderBrush}" BorderThickness="0" CornerRadius="33,33,33,33">
<Border.Background>
<LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
<GradientStop Color="White" Offset="0.0" />
<GradientStop Color="Silver" Offset="0.5" />
<GradientStop Color="White" Offset="0.0" />
</LinearGradientBrush>
</Border.Background>
<ContentPresenter Content="{TemplateBinding ContentControl.Content}" HorizontalAlignment="Center" VerticalAlignment="Center" ></ContentPresenter>
</Border>
</ControlTemplate>
</Button.Template>
</Button>