CSR for Group

HI, colleagues!

I have a code CSR

overrideCtx.Templates.Group = CustomGroup;
overrideCtx.Templates.Item = customItem;

and

function CustomGroup(ctx, group, groupId, listItem, listSchema, level, expand) {

    //var html = '<ul id="list1">' + listItem[group] + '</ul>';
    html = '<ul id="list1">' + listItem[group] + listItem;
    html += '</ul>';

    return html;
 
}


function customItem(ctx) {
    var ret = "<li><div>" + ctx.CurrentItem.Title + "</div></li>";
    return ret;
    
}

but it generates separate blocks

<ul id="list1">123</ul>
<li>
<div>123</div>
</li>
<li>
<div>123</div>
</li>

how to make the blocks were elements within the block groups like this

<ul id="list1">123
<li>
<div>123</div>
</li>
<li>
<div>123</div>
</li>
</ul>