How to retrieve which radio button within a LI is selected

<ul class="ulGroup" runat="server" id="ulGroup">
    <li>
        <input type="radio" name="grouptype" id="rdGroup1" class="css-checkbox" value="Group 1" checked="checked" runat="server" />
        <label for="rdGroup1" class="css-label radGroup1">Group 1</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup2" class="css-checkbox" value="Group 2" runat="server" />
        <label for="rdGroup2" class="css-label radGroup1">Group 2</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup3" class="css-checkbox" value="Group 3" runat="server" />
        <label for="rdGroup3" class="css-label radGroup1">Group 3</label>
    </li>
    <li>
        <input type="radio" name="grouptype" id="rdGroup4" class="css-checkbox" value="Group 4" runat="server" />
        <label for="rdGroup4" class="css-label radGroup1">Group 4</label>
    </li>
</ul>

How do I retrieve which li radio button is selected from code-behind.

When I run the code, I get the following error:

Parser Error Message: Cannot create an object of type 'System.Boolean' from its string representation 'checked' for the 'Checked' property.