How to redraw Button as a circle in XAML

<Style x:Key="btnNext" TargetType="Button">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Margin)" Storyboard.TargetName="rectangle">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Thickness>-3</Thickness>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal"/>
<VisualState x:Name="Disabled"/>
<VisualState x:Name="MouseOver"/>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="rectangle" RadiusY="25" RadiusX="25" Stroke="Blue" StrokeThickness="4">
</Rectangle>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="Margin" Value="5"/>
<Setter Property="Width" Value="50"/>
<Setter Property="Height" Value="50"/>
<Setter Property="FontSize" Value="120"/>
<Setter Property="Foreground" Value="White"/>
</Style>